Bias in dating news and historical events

acta psychologica

ELSEVIER Acta Psychologica 86 (1994) 69-87

Bias in dating news and historical events

Simon Kemp *

Department of Psychology, Unicersity of Canterbury, Christchurch, New Zealand

Received May 1993; revised October 1993

Abstract

The tendency to date relatively remote events too recently and relatively recent events too remotely was investigated in two experiments where undergraduate students dated both news and historical events. Experiment 1 obtained this pattern of dating bias regardless of whether subjects were supplied with boundaries for the extreme dates of the events. In Experiment 2, subjects commented on the strategies they used when dating events, and bias was found with all the strategies. Overall, the experiments showed little evidence that boundary effects or rounding were responsible for bias in dating either news or historical events.

1. Introduction

It is well documented that people not only are inaccurate at recalling when past events occurred but also produce characteristic biases in dating them (e.g. Brad- burn et al., 1987; Friedman, 1993; Huttenlocher et al., 1988; Rubin and Baddeley, 1989). This article is chiefly concerned with evaluating different explanations advanced for one frequent pattern of bias: the tendency to date relatively remote events too recently and relatively recent events too remotely.

A number of studies have found this pattern of bias when people have been presented with, and asked to date, events reported in the news over the previous

* I acknowledge the support of the New Zealand Department of Labour under the Restart and

Taskforce Green schemes. and the Psychology Department Research Fund Committee. I am grateful to Kim Branch for help in coding the results of Experiment 1, and to Karyn Willetts for help coding and

analysing Experiment 2. Bill Friedman, Gill Rhodes and two anonymous reviewers provided useful and insightful comments on earlier versions of the paper.

E-mail: [email protected]

Elsevier Science B.V. SSDI OOOI-6918(93)E0058-A

70 S. Kemp /Acta Psych&@ X6 (1994) 69-87

few years (e.g. Brown et al., 1985; Kemp, 1988; Lieury et al., 1980; Lieury et al., 1979). A similar result has been obtained when subjects have been asked to mark timelines rather than generate dates (Ferguson and Martin, 1983). Nor is the result restricted to the dating of news events: The same pattern of error has been found for psychologists’ recall of when colloquia took place (Rubin and Baddeley, 1989) film goers’ estimation of when they saw movies at film societies (Huttenlocher et al., 1988) the date of a survey interview (Huttenlocher et al., 1990) and diarists’ recall of the dates of events they recorded (Burt, 1990; Erickson and Jemison, 1991; Thompson et al., 1988; White, 1982). Similar bias occurs when subjects are asked to date historical events, events taking place before the subjects were born

(Kemp, 1988). Temporal error has also been found in surveys where people have been asked

questions like ‘Have you xed in the last six months?’ There is a frequent tendency to include instances of x from before the six-month period (e.g. Loftus and Marburger, 1983; Sudman and Bradburn, 1973). The bias apparent in such a response, sometimes referred to as fonvard telescoping (Loftus and Marburger, 1983), is consistent with relatively distant events being dated too recently, and it may be that dating bias and the bias in surveys should be explained by the same theory. On the other hand, as is discussed below, different sources of bias may surface in different response situations.

A number of theoretical ideas have been put forward to account for the pattern of bias found when events are dated. Perhaps the most general explanation is that the pattern results from actual compression of remembered time, so that remembered elapsed time is a power function of actual elapsed time with an exponent less than one (e.g. Bratfisch et al., 1971; Ferguson and Martin, 1983) in the same way, for example, as the loudness of sound is found to be a compressive function of sound intensity (Stevens, 1975). This theory is by no means implausible and has the advantage of apparently explaining a diverse range of phenomena, but many researchers have been reluctant to accept that the mental representation of time is nonlinear while other theories are available (e.g. Huttenlocher et al., 1988; Rubin and Baddeley, 1989). Moreover, there are empirical results which, while not necessarily inconsistent with this theory, are not actually explained by it.

Not all distant events are dated too recently and not all recent events are dated too remotely. Indeed, there are usually some events for a given subject or even a sample which are dated rather accurately. (For example, the Treaty of Waitangi or the beginning of World War I in Table 1 below, or White’s (1982) accurate recall of dates in a trip to the U.S.A.) Such dates, usually of better-known events, show no sign of time compression. It is not very parsimonious to suppose, however, that well-known events are dated in linear time while events about which one is uncertain are dated in nonlinear time.

A second problem arises from the fact that whether the age of an event is over- or under-estimated depends on the kind of event and the context in which it is presented. For example, in the three-month diary studies of Erickson and Jemison (1991) and Thompson et al. (198X), the older ‘personal’ events (2-3 months previous) were dated too recently. On the other hand, in Ferguson and Martin’s

71 S. Kemp /Acta Psychologica 86 (1994) 69-87

Table 1

Events used in Experiment 1 with their actual dates of occurrence and absolute error

Event Actual date Absolute

(year) error

Historical euents

Watt’s steam engine perfected 1765 106 James Cook first visits New Zealand 1769 30 John Wesley founds Methodist Church 1784 66 French Revolution 1789 26 Samuel Marsden celebrates first church

service in New Zealand

Battle of Waterloo

Te Rauparaha captures Kaiapoi

Treaty of Waitangi First postage (penny) stamps introduced in Britain

First four ships arrive in Lyttelton

Charles Darwin publishes The Origin of Species Gold discovered in Otago

Suez Canal opens

Alexander Graham Bell invents telephone

First cricket test (between England and Australia)

Eruption of Krakatoa

‘Jack the Ripper’ murders in London Birth of Katherine Mansfield

Women gain vote in New Zealand

Marconi invents the ‘wireless’

Einstein publishes special theory of relativity

World War I begins

James Joyce publishes Ulysses

Tutankhamen’s tomb discovered

Great Depression begins

George Biro invents ball-point pen First atom bomb exploded

Invention of the transistor

1814

1815

1830

1840

1840

1850 1859

1861

1869

1876

1877

1883 1888

1888

1893

1895

1905

1914

1922

1922

1929

1938

1945

1947

27

40

26

0 40

12

33

19

45

24

27

42

22

14

11

15

24

0

32

19

24

37

News events

Russians shoot down Korean airliner

Indian army storms Golden Temple in the Punjab

Wellington Trades Hall caretaker Ernie

Abbot killed by suitcase bomb Bomb explodes at Conservative party conference

Mrs. Indira Gandhi shot dead 41 die at European Cup final in Brussels when

English soccer fans riot ‘Rainbow Warrior’ sunk in Auckland harbour

‘Titanic’ wreck located Keri Hulme wins Booker Prize for The Bone People US space shuttle ‘Challenger’ explodes

just after take-off

Sept, 83 3.63 Mar, 84 2.42

June, 84 2.25 Ott, 84 2.67 Oct. 84 2.00

May, 85 2.96 Jul, 85 0.98 Sept, 85 1.92 Ott, 85 1.09

Jan, 86 0.96 Russians admit an accident at the

Chernobyl nuclear power station Prince Andrew marries Sarah Ferguson

Apr, 86 1.17 Jul, 86 1.00

72

Table 1 (continued)

S. Kemp/Acts Psych&&a 86 (1994) 69-87

Event

GST first takes effect Pope John-Paul II visits NZ

Mathias Rust lands plane in Moscow’s Red Square

Colonel Rambuka launches first Fijian coup

NZ wins World Rugby Cup final

Rudolf Hess dies in Spandau Prison

Rewi Alley dies in China

US Hercules buried in Antarctica for I6 years flies into Christchurch

US cruiser ‘Vincennes’ shoots down

Iranian Airbus

George Bush elected President of the USA

Death of Japanese Emperor Hirohito Vicki Buck becomes Mayor of Christchurch

Romanian President Ceausescu executed

Kiri Te Kanawa sings in Hagley Park

Richard Hadlee takes 400th test wicket

Nelson Mandela freed

Actual date Absolute

(year) error

Oct. 86 NW, 86 Apt-, X7 Apr, X7

Jun, X7

Aug, 87

Dee, X7

Jan. 8X

Jul, 88

Nov, 88

Jan. 89

Ott, x9 Dec. 89

Jan. 90

Feb. 90

Feb, 90

1.09

1.46

I.39

0.96

0.98

1.58

1.5x

139

1.79

0.66

0.84

0.17

0.33 0.09

0.25

0.17

(1983) study news events occurring in the previous seven months were dated too remotely. Kemp (1988) found that the older news events (all occurring after 1979) were generally dated too recently, but the more recent historical events (all occurring before 1950) were generally dated too remotely by the same subjects. These discrepancies are not predicted by a general time compression model.

Those theorists who are reluctant to discard linear representation of elapsed time have suggested a number of other ways in which bias in dating events might arise. A common starting point has been to recognise that assigned dates not only show bias in their mean or median values but also considerable variance which increases with the age of the event (e.g. Huttenlocher et al., 1988; 1990; Rubin and Baddeley, 1989). Bias in dating events is then seen as arising from the combination of increasing but symmetrical variance in remembered estimates and biasing effects which arise from the nature of memory or from the recall situation. Biasing effects suggested include those of boundaries, intrusions, and rounding.

I. I. Boundary effects

Boundary effects arise when the dates to be estimated are perceived as being restricted, so that the subject perceives an event can not have occurred before a distant boundary and not after a recent boundary (often the time of the experiment). As Huttenlocher et al. (1988) and Rubin and Baddeley (1989) have shown, the effect of these boundaries, in combination with variance in the remembered time of the trace, is to predict that older events should be dated too recently and very recent events too remotely. For example, consider someone who has kept a diary for the last twelve weeks, and who is now asked to date a particular event

S. Kemp /Acta Psychologica 86 (1994) 69-87 73

from near the beginning of the period. If the actual time of the event is subject to error, the event may be ‘remembered’ either as closer or more distant than twelve weeks ago. The subject’s knowledge that the event took place in the previous twelve weeks sets a boundary on more distant recall, but not on more recent recall. Thus, on average, events near the distant boundary should be recalled as too recent. A similar argument suggests very recent events might be recalled as too remote, although this effect should be smaller because these events should be more accurately remembered.

Huttenlocher et al. (1988) and Rubin and Baddeley (1989) found strong evidence for boundary effects when subjects were asked to recall clearly bounded events: For example, the films recalled by Huttenlocher et al’s (1988) subjects were shown in university quarters and the beginnings and ends of those quarters emerged as distinctly perceived boundaries.

Boundaries, then, can produce dating bias. More questionable, however, is whether they are the major source of bias in most dating studies. An obvious question here is: How often are distinctive boundaries available in dating studies? The answer (excluding the fact that the time of the study necessarily constitutes a recent boundary) seems to be: surprisingly often. In diary studies, for example, the beginning and end of diary keeping form obvious boundaries. Studies where subjects have been asked to date news events do not appear to have a clear distant boundary, but in these studies boundaries have often been supplied by the experimenter (Brown et al., 1985; Ferguson and Martin, 1983; Kemp, 1988).

1.2. Intrusions

When subjects are asked to recall instances of a class of events from a certain period and then to date them, the possibility of intrusions from events outside (usually before) the nominated period arises. Rubin and Baddeley (1989) have discussed this error, which may contribute to forward telescoping, and shown that it can contribute to the characteristic dating bias. However, this source of bias is not present where the experimenter rather than the subject generates the events to be dated, as is usually the case in dating studies (and Experiments 1 and 2 below).

1.3. Rounding

Huttenlocher et al. (1990) suggest that the way in which we round periods of time contributes to dating bias. They found that subjects tended to round periods of elapsed time to prototypic values, from 33 to 30 days, for example. A general tendency to round dates or periods of elapsed time will not in itself produce a dating bias, although it will produce inaccuracy. In Huttenlocher et al.‘s (1990) model, however, the bias results from the combination of two features of the way the rounding is supposed to take place. The prototypic values are assumed to become more sparsely distributed with the increase of elapsed time, and subjects are assumed to choose the nearest prototype.

For example, take the prototypic values 7, 10, 14, 21, and 30 days. An assigned value of 14 days should occur for all days between 12 (midway 10 and 14 days) and


17.5 days (midway 14 and 21). Then, if actual days are evenly distributed, a tendency to date too recently will result. Similar reasoning applies to dates provided more distant prototypic dates are sparser. Note that both the assump- tions are necessary to produce bias, and that, for example, an increase in error or a sparser distribution of prototype dates with elapsed time is not in itself sufficient to produce bias.

1.4. Ouerrjiew of the experiments

The following experiments investigated the importance of boundary effects and rounding as sources of bias in dating events, concentrating chiefly on the question of whether boundary effects are the sole or principal causes of dating bias. This concentration was suggested by the existence of well-developed and articulated theories (Huttenlocher et al., 1988; Rubin and Baddeley, 1989) which seek to explain dating bias as the result from this effect, and for which there is at least prima facie evidence.

Experiment 1 considered the effect of experimenter-supplied boundaries on dating. In Experiment 2, subjects dated events and reported on their use of boundaries and their knowledge of the events.

Most research on dating events has explored how people date events that they may be expected to have heard about at the time, so that the age of the event should equal the age of the oldest memory of it. It is also interesting, however, to investigate how people date historical events (Kemp, 1988; Smith and Tomlinson, 1977), since for these events there is generally no correlation between the age of the event and the age of the memory. If the dating of historical and comparatively recent news events demonstrate similar biases, processes which are not linked to the age of the event are suggested. On the other hand, differences in result patterns would suggest that factors related to the age of the memory or information also encoded at the time (e.g. where you were when you heard about a particular event) may be at work, and ‘distance-based’ explanations (Friedman, 1993) would be supported. Both experiments investigated the dating of both news and historical events.

2. Experiment 1

This experiment examined the contribution of a boundary effect to bias in dating news and historical events by having subjects date events either where the experimenter supplied boundaries or where no boundaries were supplied.

2.1. Method

Questionnaires and procedure Two different questionnaires were used, one of which supplied date boundaries

while the other did not. Both questionnaires required subjects to date the same 56 past events.


Events were of two kinds: There were 28 historical events (also used by Kemp, 19SS>, 1 all of which actually occurred between 1750 and 1950; and 28 news events, all of which actually occurred between September, 1983 and March, 1990. The bulk of the questionnaires consisted of brief descriptions of the events and a space for the subject to write a date alongside. The brief descriptions and actual dates of the events used are shown in Table 1.

The 56 events were made into a booklet. Each page contained either 14 news or 14 historical events. A single random placement of the events on each page was used, but the pagination was randomly varied between booklets.

The cover page of each questionnaire requested subjects to date all the events in the questionnaire. Boundary questionnaires also stated that the events to be dated were of two kinds, historical, occurring between 1750 and 1950, and recent, occurring between September, 1983 and the present. Historical events were to be dated by supplying the year of occurrence and news events by supplying both month and year. Each page of events in the boundary questionnaire was headed by a statement that the events on that page were either recent or historical and reiterated the boundary dates.

Control questionnaires stated on the cover page that events were to be dated either by supplying the year or the year and month of occurrence, and that a brief statement at the top of each page would indicate which kind of date to supply.

Supplying a date for each event made each subject eligible for a $100 lottery, which was drawn at the completion of the experiment. Questionnaires were distributed and completed in scheduled laboratory times in June 1990. Receipt of a boundary or control questionnaire was randomly determined.

Subjects Eighty subjects, 31 of them male, participated, 41 completing boundary and 39

control questionnaires. Their ages ranged from 18 to 45, averaging 21.7. All were enrolled in advanced undergaduate courses in psychology; none had been previously asked to date past events.

2.2. Results and discussion

For both types of event the dependent variable was the estimated year of occurrence. For historical events this was always a whole number, e.g. 1884. For news events a decimal value was calculated for each month, equalling the proportion of the year elapsing in the middle of the month. For example, May was coded as 4.5 months into the year, or .38 in decimal form. May, 1988 thus became 1988.38. When only the year of a news event was given, which happened with a

1 Since the same set of historical events is also used in Experiment 2, the reader might wonder whether

the pattern of results is restricted to these 28 events. A brief historical dating study carried out in

Germany with completely different events also produced the pattern of more remote historical events being dated too recently and more recent ones too remotely.

76 S. Kemp /Actu Psychologica 86 (1994) 69%N7

83 04 85 06 87 88 89 90 91

ACTUAL DATE Fig. 1. Median estimated date of news events as a function of actual date for boundary and control

questionnaires in Experiment 1. The equations of the regression (solid) lines are: Control. Est.

Date = 0.629 (Act. Date)+32.927 (r’ = 0.71); Boundary, Est. Date = 0.63X (Act. Date)+32.132 tr’ =

0.72).

median 4 out of 80 subjects per event (range 2 to 51, this was coded as .5 of that year.

Fig. 1 shows the relationship between median estimated date of the news events and actual date for each type of questionnaire. Fig. 2 shows the results for the historical events. Note, first, that the pattern of systematic error reported by previous researchers is found here for both news and historical events and for both control and bounded questionnaires: More recent dates are dated too remotely, particularly for the historical events, and more remote dates too recently, resulting in regression line slopes of less than 1. * Second, similar systematic bias appeared in response to both boundary and control questionnaires.

Regression of estimated on actual date was also carried out for each individual subject for news and historical events separately. The presence of occasional very large or negative individual regression slopes, the result often of one very unusual

’ The use of regression slopes less than one to argue for dating bias raises two questions. Firstly, might

lower slopes result from attenuated correlations between estimated and actual date? However, low

correlation between a Y and an X variable will only bias the slope of the regression line if there is error on the X variable (e.g. Stigler. 1979). That is not the case here. Secondly, might the general

tendency for error to increase with the age of the event bias the slope estimates? There is no reason for this to occur, however, if the error is symmetrical, which is essentially the question under investigation.


0,' / Boundory

1750 -,/' /' 1750 1850 1950

ACTUAL DATE

Fig. 2. Median estimated date of historical events as a function of actual date for boundary and control

questionnaires in Experiment 1. The equations of the regression (solid) lines are: Control, Est.

Date = 0.690 (Act. Date)+585.9 (r* = 0.80); Boundary, Est. Date = 0.691 (Act. Date)+583.4 (r* =

0.72).

date estimate (e.g. the battle of Waterloo dated as 1066), suggested the use of nonparametric tests in analysing differences between the slopes. 3

For the historical events, the median individual slope was 0.70 (Quartile deviation CQD> = 0.20) for the control and 0.63 CQD = 0.12) for the boundary questionnaires; this difference was not significant by a Mann-Whitney U test (2 = 1.46, n.s.1. For the news events, the median individual slope of 0.74 <QD = 0.21) was significantly higher for the control than the boundary questionnaires (median = 0.55, QD = 0.13; Mann-Whitney ZJ, z = 2.52, p < 0.05). Thus, present- ing the subjects with boundaries significantly affected their dating of news but not historical events. Note, however, that even with the control questionnaires the median individual slopes were less than one, indicating that boundary effects are

3 It might be argued that these unusual date estimates should be excluded when calculating individual

regression slopes. Consequently, the analyses using individual slopes in Experiments 1 and 2 were also performed after regression that excluded the two data points with the highest residuals (regardless of sign). Because the deviant dates were mostly too early, their exclusion tended to lower the slopes and

hence increase apparent dating bias. This increase, in fact, suggests that excluding extreme dates actually biases the results, hence unmodified slopes appear in the text. However, the exclusion of

deviant dates affects the results of only one of the significance tests performed on the regression slopes. (The modified slopes for news events were not significantly different [Mann-Whitney ZJ, I = 0.63, n.s.1

between control and boundary groups in Experiment 1.)


Table 2

Numbers of subjects placing all their dates within the boundaries or at least one outside the boundaries

for each questionnaire type and type of event

Event type/

Questionnaire type All inside

boundaries

At least one

outside

boundaries

News events

Boundary

Control

Historical events

Boundary

Control

25 16

6 33

30 11

11 28

not the sole cause of dating bias. In fact, of the 39 control subjects, 30 obtained slopes less than one when dating news events and 33 obtained slopes less than one when dating historical events: Both numbers are significantly greater than would be expected if there were no bias (sign test, p < 0.01).

Table 2 shows the numbers of subjects completing boundary and control questionnaires divided up by whether one or more of their dates went outside the set boundaries. The instructions of the boundary questionnaire did significantly influence the likelihood of staying within the bounds set for both the news, (x’[l, II = SO] = 17.50, p < 0.001) and the historical events (x2[1, II = SO] = 16.18, p < 0.001).

On average, historical events were dated too recently by 7.3 years for the control and 5.2 years for the boundary groups (t(27) = 1.02, n.s.>. News events were dated too recently by 0.63 years for the control and 0.58 years for the boundary groups (t(27) = 0.52, n.s.1. Overall, then, events were dated too recently: this was particularly marked for the news events where 23 of the 28 events were dated too recently by both the control and boundary groups. Eighteen of the historical events were dated too recently by both groups. Similarly, the neutral points as revealed by the regression lines of median estimated date on actual date, the point for each condition where there is no overall tendency to date either too recently or too remotely, are more recent than the midpoints of the actual dates for all conditions. For historical events, the neutral points were 1888 for the boundary and 1890 for the control condition. For news events, it was the beginning of October, 1988 for both conditions.

The results for the news events are interesting in that they are not what would be expected if the imposition of boundaries was the major cause of dating bias. The control group was given no remote boundary, although, as for the boundary group, the time of the experiment provided a recent boundary. Under these circumstances, the control group should tend to date events more remotely than the boundary group and too remotely overall. Neither of these expectations was fulfilled.

On average, each news event was exactly dated (i.e. month and year correct) by 5.5 subjects (SD = 6.22) and each historical event by 7.9 subjects (SD = 15.5). The

S. Kemp/Acts Psychologica 86 (1994) 69-87 79

last column of Table 1 shows the median absolute errors of the date estimates of each event taken over both types of questionnaires. The average of these median absolute errors was 1.35 years (SD = 0.88) for the recent and 27.2 years (SD = 21.6) for the historical events. There was no significant difference in absolute error between the boundary and control groups for either the recent (t(27) = 0.91, ns.) or historical (t(27) = 0.94, n.s.1 events. More recent events were dated more accurately for both kinds of event: the Pearson product moment correlations of median absolute error with actual event date were -0.81 (p < 0.01) for the news and -0.55 (p < 0.05) for the historical events.

In conclusion, while Experiment 1 found evidence for boundary effects, these were not the sole or major source of dating bias. This conclusion arises because the majority of the subjects in the control as well as the boundary group showed dating bias for both historical and recent events. It is also supported by the overall too recent dating of news events in the control condition.

3. Experiment 2

Experiment 1 showed that the experimental manipulation of boundaries had little effect on dating bias for either news or historical events. This experiment, however, leaves open the possibility that subjects may generate their own boundaries in the course of dating events. Hence, Experiment 2 required subjects to date either historical or news events and afterwards to report on their use of boundaries.

To investigate the effects of rounding (cf. Huttenlocher et al., 19901, the dating bias obtained with possibly rounded dates was compared to that obtained with the other dates.

Finally, to investigate whether dating bias was dependent on other processes used in dating events, subjects were asked to rate their knowledge of the different events and to indicate whether they had used landmarks.

3.1. Method

Questionnaires and procedure Two different questionnaires were used, one requesting the dating of 28 news

events, the other the dating of 28 historical events. The historical questionnaires presented the same events as Experiment 1. Two of the news events actually occurred in 1985 (the earliest in July), and 2 in 1992 (both in February). The other 24 news events were taken at the rate of 4 per year from 1986 to 1991. The overall median date of the news events was December, 1988. The experiment was carried out in June, 1992.

On the cover page, subjects were asked to date each event and to rate their knowledge of it on a scale from 0 to 9. ‘0 means you know or remember absolutely nothing about the event (except what you are reading now); 9 means you know a great deal about it.’ Historical event questionnaires requested the year of the


event, news event questionnaires the month and year. Subjects were told to date all the events, even those they had never heard of before.

Events were presented three to a page, with one event on the last page. A brief description of each event was followed by spaces for the date and knowledge rating, a question asking whether another event had been used in dating the event, and a further question about when the other event occurred. Different random event orders were used.

After the last event, there were questions about the use of boundaries. Subjects were asked whether they had the impression that all the events fell into a certain time period, and, if so, what they thought the boundaries of the time period were (i.e. what they thought the remotest and most recent events contained in the questionnaire were). They were also asked if whilst dating the events they had ever used an idea of what the boundaries might be to help them decide that a particular event was more remote or more recent than they originally felt. Finally, they were requested to indicate each event for which they had done this.

Subjects completed the questionnaire under supervision in scheduled laboratory times. A lottery for $25 per group of 20-30 subjects was conducted immediately after the group had completed its questionnaires.

Subjects Fifty subjects, 33 of them female, with ages ranging from 18 to 44 years

(median = 20), completed historical questionnaires. They were all enrolled in an advanced undergraduate course in psychology. Seventy-five (52 female; age range 17 to 45 with median age 19) enrolled in an introductory course in psychology, completed the news questionnaires. No subject had previously participated in research on dating events.

3.2. Results and discussion

When the median estimated years of occurrence were regressed on the actual dates, the equation for the recent events had a slope of 0.83 (r2 = 0.94) and that for the historical events had a slope of 0.66 (r 2 = 0.65). Regression lines were also calculated for each subject: The median slope for the recent event questionnaires was 0.90 (median r2 = 0.36) and that for the historical events 0.71 (median r 2 = 0.36). Fifty- one of the 75 subjects dating recent events recorded slopes of less than 1; 44 of the 50 subjects dating historical events. Both numbers are significantly greater than would be expected by chance (sign test, p < 0.01). In summary, both recent and historical events were subject to dating bias.

Fourteen of the historical and 24 of the news events were assigned median dates that were too recent. As in Experiment 1, the neutral points derived from the aggregate median results, 1892 for the historical events and October, 1991, were more recent than the midpoints of the actual dates for both types of event, as were the median neutral points, 1894 for the historical and September, 1991 for the news events, calculated from regression lines fitted to the results of individual subjects. Thus, as in Experiment 1, there was an overall tendency to date events too recently.


Boundaries Fifty-seven of the 75 subjects completing news event questionnaires, and 40 of

the 50 completing historical event questionnaires indicated that they had the impression that the events fell into a certain time period, and the median remotest and most recent boundaries given by these subjects were May, 1982 and 1992 (month not given) for the recent events and 1750 and 1950 for the historical events. Thus most subjects did gain an impression of boundaries and the impressions were reasonably accurate. On the other hand, the impressions did not necessarily act as barriers to dating outside the time period: Of those who had gained the impression of a time period, 30 of the 57 news events subjects and 22 of the 40 historical event subjects dated at least one event outside the boundaries they had given.

Fourteen news event subjects and 17 historical event subjects indicated they had modified their estimated date for at least one event by considering boundaries. When individual events were considered, the number of subjects claiming to have used boundaries in dating each event ranged from 2 to 8 (median = 6 subjects) for the news events and from 3 to 9 (median = 5.5 subjects) for the historical events. Thus the conscious use of boundaries to modify event dating does not seem to have been a frequent strategy.

The effect of apparent use of boundaries on dating bias as measured by the slope of the regression equation was investigated, firstly, by considering whether or not the slopes of individual regression equations varied with whether or not the subject had the impression of the events falling within a certain time period. Mann-Whitney U tests showed no significant effect of this impression on the slope for either the news (z = 0.03, n.s.1 or the historical (2 = 1.16, ns.) events. A similar analysis showed no effect on the slopes of either news (2 = 0.08, n.s.> or historical events (z = 1.18, n.s.> of whether subjects claimed to have used their impression of the boundaries to modify the date assigned to any event.

Table 3 shows the slopes obtained when the median estimated boundary and non-boundary dates were regressed on the actual date according to three different criteria for determining when boundaries were used: whether or not the subject had an impression of a time period; whether the subject claimed to use the

Table 3 Slopes of the line regressing different medians of estimated on actual dates in Experiment 2

News events Historical events

Impression of time period

No impression

0.84 (0.95) 0.66 (0.70) 0.84 (0.55) 0.63 (0.53)

Used boundaries at least once

Did not use at all

Used boundaries on these events Did not use on these events

Knowledge rating of 0 Knowledge rating of 1 or more

0.81 (0.93) 0.77 (0.69)

0.84 (0.92) 0.45 (0.32)

0.86 (0.82) 0.63 (0.57) 0.84 (0.92) 0.66 (0.67)

0.61 (0.32) 0.61 (0.56) 0.79 (0.86) 0.70 (0.66)

Note: r2 given in parentheses alongside the slope.

82 S. Kemp /Acta Psychologica X6 (1994) 69-87

impression to modify any date; and whether, according to a subject’s claim, a particular date estimate was obtained with the help of a boundary or not. (Note that, under the third criterion, median estimates for different events are generally based on different groups of subjects.) The table indicates that for the aggregate as well as the individual results there was no marked effect on dating bias, and that, no matter which criterion was employed, events that were not dated with boundaries also showed dating bias.

The presence of significant dating bias was investigated by whether significantly more than half the subjects recorded line regression slopes less than one on a test of proportion (p = 0.05, one-tail) for subjects having either an impression of the time period or not or using boundaries at least once or not. Significant dating bias according to this criterion was found for news events for those subjects who did obtain an impression of boundaries (40 with slopes less than 1 out of 56) and for the subjects who claimed never to have used boundaries at all (41/59). No significant dating bias was found for subjects who did not obtain an impression of boundaries (1 l/17) or for subjects who claimed to use boundaries (10/14). For the historical events, significant dating bias was found both for those who did (35/39) and those who did not (9/10) obtain an impression of boundaries; and for those who claimed to use boundaries at least once (16/17) and those who did not (28/32). These results are in general agreement with Table 3 in showing that, particularly for the historical events, dating bias occurred regardless of whether subjects formed an impression of or used boundaries.

Overall, the results relating to boundaries showed that the subjects could and often did construct an idea of the approximate period covered by the dates they were asked to date, and that they occasionally did use this information to help them date unknown events. However, this kind of process often did not lead to the formation of definite boundaries, nor does it appear to have been the sole or even a major cause of dating bias in the experiment.

Absolute errors ad knowledge ratings

Absolute errors were calculated for each subject and event, and the median absolute error calculated for each event. These ranged from 0.16 of a year to 3.09 years (median = 1.00 years) for the news events, and from 0.5 of a year to 91.5 years (median = 32 years) for the historical events. For the news events, the median absolute error correlated -0.66 (p < 0.01) with the actual date and -0.75 (p < 0.01) with the average knowledge rating. For the historical events these correlations were -0.69 (p < 0.01) and -0.61 (p < 0.01) respectively. Thus the more remote events and those about which subjects claimed less knowledge were less accurately dated.

Average knowledge ratings were also highly variable between the events, ranging from 0.57 to 4.68 (median = 2.84) for the different news events and from 0.38 to 4.74 (median = 1.30) for the historical events. The average knowledge rating correlated 0.29 (n.s.) with the actual date for the news and 0.22 (ns.) for the historical events. (These results partly reflect my decision to present well and less well-known remote and recent events.)

S. Kemp /Acta Psycholvgica 86 (1994) 69-87 83

The effect of average knowledge on dating was investigated by first calculating the median estimated date for each event for those subjects indicating 0 knowledge (indicative of guessing) and for those subjects indicating knowledge greater than or equal to 1. (The numbers of subjects scoring 0 varied greatly over the different events.) Separate regressions of median estimates on actual dates were then performed for the two groups of medians for each type of event, and these are shown in Table 3. The results suggest that dating bias was more pronounced when little was known about the event, but was still present for those events about which there was some knowledge. This is unsurprising, because events about which more knowledge was claimed were also more accurately dated. Note also, because the correlations of estimated and actual date are greater than zero, that subjects could approximately date events about which they claimed to know nothing.

It might be expected that respondents with higher average knowledge ratings would also be more likely to obtain an impression of boundaries or to use them. Furthermore, their boundaries might be more precise. However, there was no significant difference in average knowledge ratings between those who did or did not gain an impression of boundaries for either the news (t(71) = 1.69, n.s.1 or the historical events (t(48) = 0.36, n.s.1, nor between those who did or did not claim to have used boundaries in dating events (news: t(71) = 1.61, ns.; historical: t(48) = 0.01, n.s.>. Nor, for either news or historical events, were there significant (p < 0.05) correlations between the average knowledge ratings and the absolute difference between the time of the remotest (or most recent) event and the stated remote (or recent boundary).

Rounding Analyses were performed on the historical and news dates to investigate

whether rounding occurred and, if it did, whether it was responsible for dating bias. For the historical events, dates were segregated into those ending in 0 (e.g. 1880, 1750) and those ending in some other digit. Taken over all the subjects and events 59 percent ended in 0, ranging from 6 percent for the beginning of World War I to 80 percent for Watt’s steam engine. Moreover, 15 percent of all the events were assigned dates ending in 00 or 50. These percentages are clearly greater than would be predicted by chance and indicate that many of the dates ending in 0 derived from rounding.

In the next stage of the analysis, the median of all the dates ending in 0, the median of those ending in some other digit, and the median of those ending in 00 or 50 were calculated for each event and these three sets of medians regressed on the actual dates. If dating bias were associated with rounding to any great extent or were in some way caused by it, the slopes of the regression equations would be steeper for dates not ending in 0. However, the actual slopes were 0.69 (r* = 0.65) for those ending in 0, 0.58 (r* = .54) for those ending in some other digit, and 0.65 (r2 = 0.46) for those ending in 00 or 50, results quite contrary to the prediction.

For the news events, it was surmised that rounded dates might be more likely to be a June date, suggesting that subjects thought ‘a year ago’ or ‘3 years ago’ etc. Alternatively, no month might be given at all. Only 7.4 percent of all the dates (9.6


percent of those assigned a month) were in June, suggesting that June dates did not generally arise from rounding. On the other hand, 31 percent of all the news events were dated without months. There was a tendency for individual subjects to either always or very rarely indicate a month: Thus, the percentage of monthless dates ranged only from 24 to 35 across the different events. 4

When the median dates of all the monthless dates and the median date of all those assigned months were separately calculated for each event and regressed on the actual dates, a slope of 0.81 (r* = 0.71) was obtained for the monthless dates and 0.85 (r2 = 0.93) for those assigned months.

A separate analysis of the months assigned to events showed that overall 11.8 percent (ranging from 4.0 to 40.4 percent over the different events) of the month estimates were accurate, while 29.1 percent of the estimates (ranging from 16.0 to 54.4 percent) were either accurate or of the following or preceding month. These percentages are little higher than expected by chance (8.3 and 25 percent respectively), so clearly the months of occurrence of these news events were generally poorly known. Taken over the different events, there were significant Pearson correlations between the proportion of accurate month estimates and the average knowledge rating (r(26) = 0.44, p < 0.05) and the median absolute error (r(26) = -0.39, p < 0.05). Similar correlations were obtained for accurate plus or minus a month estimates (average knowledge rating: r(26) = 0.49, p < 0.01; median absolute error: r(26) = -0.45, p < 0.05).

Overall, subjects did round dates, particularly for historical events, but rounding was not a major cause of dating bias in this experiment. For the historical events, dating bias appears to have been slightly greater with dates that do not suggest rounding than with dates that do. For the news events, the monthless dates were slightly more subject to dating bias than those assigned a month as the hypothesis would predict, but dating bias was also present in the 69 percent of dates that did contain months.

Landmarks The number of subjects claiming to have used landmarks ranged from 0 to 17

(median = 5 subjects) over the different news events, and from 0 to 12 (median = 2 subjects) over the historical events. Thus landmarks were rather infrequently used. Slopes of the regression equations for median estimated dates of landmarked and non-landmarked events (generally based on different subjects for each event) are shown in Table 3. The table suggests that landmarking reduced the extent of dating bias, particularly for the news events, but it did not eliminate it.

Landmarks were used to date news events more often than historical events. Of the total 76 historical events dated through the use of landmarks, 14 were dated with landmarks that preceded the target event, 30 with landmarks that were

’ The proportion dated without months was rather higher in this experiment than Experiment I. Subjects in the earlier experiment may have inferred from their instructions that including months was

a precondition for lottery eligibility.


simultaneous with the target, and 32 with landmarks that followed the target. Of the total 165 landmarked news events, 14 were dated with the aid of preceding landmarks, 106 with simultaneous landmarks (often by reference to the subject’s own living circumstances), and 4.5 with landmarks that occurred after the target. The relatively heavy use of simultaneous landmarks is unsurprising, but it is noteworthy that for both the recent and the historical events there was significantly (sign test, p < 0.01) more use of landmarks that followed than of landmarks that preceded the target event.

4. General discussion

In both experiments there was little difference in the dating bias obtained between bounded conditions and control or non-bounded conditions, regardless of whether the bounds were supplied by the experimenter or generated by the subjects. Moreover, the control condition of Experiment 1 and subjects who claimed to have formed no impression of boundaries or not to have used them in dating events showed significant bias that was similar in magnitude to that obtained with conditions or subjects who did use boundaries. Nor were either the experimenter-supplied or subject-generated boundaries impervious. In sum, boundary effects were not the sole or a major cause of the dating bias found in the present research, although it is possible they are a major cause of dating bias when there are natural boundaries (e.g. Huttenlocher et al., 1988).

Experiment 2 found rounding of dates often took place, as suggested by Huttenlocher et al. (19901, but rounded dates of historical events were no more subject to bias than dates that did not appear to be rounded. Moreover, dates which showed no evidence of rounding were also subject to bias.

The present research suggests that dating bias is too pervasive a phenomenon to be entirely explained by either boundary effects or rounding. This pervasiveness is emphasised by research showing that the use of landmarks to help date events does not eliminate dating bias (Kemp and Branch, 1992). It is probable then that the process or processes responsible for dating bias are general rather than dependent on a particular dating strategy or experimental paradigm.

The detailed examination of processes other than boundaries or rounding that might give rise to dating bias goes beyond the scope of the present paper. However, one possibility is that events are often dated by association with other events for which there is some kind of dating information available. When the date of an event is not known with any degree of accuracy, a search for other associated events whose dates are at least approximately known may take place. This search might, as in the case of landmarking, be accessible to introspection (e.g Baddeley et al., 1978; Brown, 1990; Loftus and Marburger, 1983; Kemp and Branch, 19921, but need not be. Such events might be other news or historical events or personal to the rememberer, and they might be associated in a variety of different ways. Brown (1990), for example, has shown political events tend to be associated together.

86 S. Kemp /Acts P.qxhologica 86 (19941 69-87

On average, we would expect these associated events to be later than the events to be dated, except where the events to be dated are very recent. This is because earlier events, or the associations of the target event with them, are more likely to be forgotten or unavailable as the age of the event increases. This model is consistent with the tendency found in Experiment 2 for landmark events to be more recent than the events they are used to date, as well as with the overall bias to date events too recently.

A final issue concerns the dating of historical events. The experiments reported here do show differences in the way news and historical events are dated. Historical events in Experiment 1 were more likely to be exactly dated. Experiment 2 showed landmarks were less likely to be used to date historical events, probably because personal landmarks are rarely available (Kemp and Branch, 1992).

Nevertheless, there were also overall similarities in the way historical and news events were dated. Both consistently showed systematic dating bias, and for neither type of event were boundaries a key factor in this bias. These similarities indicate that the biases and other results obtained here were not primarily the outcome of factors related to the age of the memory or temporal factors encoded with the memory. Thus, for example, the accessibility principle proposed by Brown et al. (1985) is unlikely to have been the major contributor to the dating biases (a result perhaps foreshadowed by its apparent failure to operate in some other studies, e.g., Burt, 1992; Kemp, 1988; Means and Loftus, 1991; Thompson et al., 1988). A more speculative point is that the way a society collectively remembers its history may be similar to the way an individual remembers the events of his or her own life.

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Bias in dating news and historical events

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