It is not easy to establish links between environmental exposures and health outcomes using individual level data • Problems for obtaining measures of dose levels

for individuals • There is also often the difficulty to deal with long

latency times between exposure and diagnosis. (eg cancer)

Ecological study designs provide one way of avoiding these difficulties and studying the effects of environmental exposures on different health outcomes.

Definition: observational study with group (instead of individual) being unit of observation (unit of analysis)

Types:• Exploratory study• Multiple group comparison study (geographical

study) • Time series study

The simplest ecological study Observing differences in rates of disease

in different geographical areas No exposures are measured and,

generally, no formal data analysis is used. The objective is to search for spatial

patterns that might suggest an environmental etiology or a special etiologic hypothesis.

• Ex1: Age-adjusted cancer mortality rates in the US by county for the period 1950-69;

• Difference statistically significant for oral cancer in geographic patterns by sex

- Men: mortality rates >in the urban Northeast tobacco smoking and alcohol consumption

- Women: mortality rates >in the Southeast hypothesis: snuff dipping • confirmed by a case-control study

• [Morgenstern H., AJPH, 1982,72,12, 1336-1344]

Observe the association between the average exposure level and the disease rate among several groups.

Eg:• Emile Durkheim, who investigated suicide in

western Europe during the 19th century.

• The data on four groups (religion and suicide rate) of Prussian provinces between 1883 and 1890 studied.

• Result: Protestants were 7½ times as likely to commit suicide as were other residents (most of whom were Catholic)

Observe the relationship between the change in the average exposure level (or intervention) and the change in the disease rate for a single population

With time trend studies involving a sudden change in exposure (e.g: the start of an intervention program): • compare the slope in the disease trend before and

after the intervention. With time trend studies involving a gradual

change in average exposure level:• must compare trends in both variables.

Frequently used in environmental epidemiology where it is difficult to measure exposure at the individual level

Generally use pre-existing data, which have been collected for other purposes and which can be available on large populations• there are many sources of environmental data, from

environmental surveillance systems which can be used (air pollution, drinking water quality, temperatures…)

Can also be used for studyng infectious diseases, especially in surveillance, in order to detect signals which can be alerts

Also useful to evaluate the impact of intervention programs or policies

Data on many variables (e.g., behaviors, or medical hx) may not be available at the population level, and the results of ecologic analyses are subject to certain limitations.• unlikely to give reliable and accurate estimates of risk but

they can be useful for the purposes of formulating hypotheses, that may then be examined by other methods.

Ecological studies look for associations between the occurrence of disease and exposure to known or suspected causes.

However, the unit of analysis is not an individual but a population / a group of persons

Disease rates and exposures are measured in each population and their relation is examined

The principle of this study design isTo identify changes in the frequency of an health

event in space and/or time

To relate these changes with environmental factors

The 2 major objectives of ecological studies are:

1) to generate or to test etiologic hypotheses, in order to explain disease occurrence;

2) to evaluate the impact of intervention programs or policies (if we have the knowledge of disease etiology).

Use of agregated data and not the individual data The statistical unit corresponds to a group of

persons• Levels of exposure are not measured individually ; but

use mean exposures in the population • Health outcome - use the frequency of the disease in the

statistical unit The principle is to look if there is an association

between • variation of exposure levels and • variation of health indicators

For example: • to look for an association between mean

chlorination levels in tap water in the district and the incidence rate of bladder cancer in different geographical areas (geographical study), or;

• to look for an association between daily levels of air pollution and the daily numbers of hospital admissions for respiratory diseases (time series study)