Descriptive Study Designs

• POPULATIONS (GROUPS)

--- Ecologic (aggregate) studies

• INDIVIDUALS

--- Case reports--- Case series--- Cross sectional surveys

Ecologic Studies• Measures that represent characteristics of

entire populations are used to describe disease and to postulate causal associations.

• Measure of interest is correlation between exposure rates and disease rates among different groups.

• Correlation coefficient (denoted as r)Range of r is from –1.0 to 1.0R evaluated in relation to difference

from 0.

Ecologic StudiesStrengths:• Cheap, quick, and simple (generally

make use of secondary data)

Limitations:• Cannot link exposure-disease

relationship at the individual level

• Uses average exposure levels rather than actual levels of exposure

• Inability to control for confounding factors

EXAMPLE: Country A: Country BPrevalence-Hypertension 30% 20%Average Salt Consumption Moderate Low

Country A Country BPerson Salt Intake Hyp. Salt Intake Hyp. 1 1 Yes 1 Yes 2 1 Yes 1 Yes 3 1 Yes 1 No 4 3 No 1 No 5 3 No 1 No 6 3 No 2 No 7 3 No 2 No 8 5 No 2 No 9 5 No 2 No 10 5 No 2 No

Avg. 3.0 30% 1.5 20%

Ecologic Studies

The “Ecologic Fallacy”:

• Erroneous conclusions based on grouped data

• Patterns observed on the aggregate level are not observed on the individual level

0

10

20

30

0 0.2 0.4 0.6 0.8 1

Suicide Rateper 100,000

Proportion Protestant

In 1951, Durkheim, reported an association between the rate of suicide and proportion of four Prussian provinces of Protestant religion.

How might the ecologic fallacy be responsible for this association?

0

10

20

30

0 0.2 0.4 0.6 0.8 1

Suicide Rateper 100,000

Proportion Protestant

As non-Protestants in each province became more in the minority, these individuals, rather than Protestants, may have been more likely to commit suicide.

Case Reports

• Careful and detailed report by one or more clinicians of the profile of a single patient

• Document unusual medical occurrences

• Can provide clues in identification of a new disease or adverse effects of exposures (i.e. a case report gave the clue OC use increases the risk of venous thromboembolism)

Case Reports

Limitations:

• No appropriate comparison group• Cannot be used to test for presence

of a valid statistical association• Since based on the experience of

one person:--- presence of any risk factor

may be purely coincidental--- not a true epidemiologic

design

Case Series (individual data)

• Description of clinical/epidemiologic characteristics of a number of patients with a given disease

• Collection of individual case reports occurring within a fairly short period of time

Case Series

Strengths:

• Used as an early means to identify the beginning or presence of an epidemic

• Can suggest the emergence of a new disease (i.e. AIDS)

Case Series

Limitations:

• Lack of an appropriate comparison group

• Cannot be used to test for presence of a valid statistical association

• Not a true epidemiologic design

Cross-Sectional Studies

• Snapshot of the health status of populations at a certain point in time

• For each subject, exposure and disease outcome are assessed simultaneously (hence also called a “prevalence study/survey”)

• Compare prevalence of disease in persons with and without the exposure of interest

Cross-Sectional Studies

dc

Not

Exposed

baExposed

No DiseaseDisease

a / (a + b) Prevalence Ratio (PR) = ------------

c / (c + d)

Cross-Sectional Studies

45050

Normal/

Low

400100High

No CHDCHDCholesterol

100 / (100 + 400) 0.2Prevalence Ratio = --------------------- = ---- = 2.0

50 / (50 + 450) 0.1

Cross-Sectional Studies

100 / (100 + 400) 0.2Prevalence Ratio = --------------------- = ---- = 2.0

50 / (50 + 450) 0.1

Interpretation: In this study population, the

prevalence of CHD is 2 times higher among

those with high cholesterol, compared to the

prevalence in those with normal or low

cholesterol.

Cross-Sectional StudiesStrengths:• Provides prevalence estimates of

exposure and disease for a well-defined population

• Easier to perform than studies that require follow-up (hence relatively inexpensive)

• Can evaluate multiple risk (and protective) factors and health outcomes at the same point in time

Cross-Sectional Studies

Strengths:• May identify groups of persons at

high or low risk of disease

• Can be used to generate hypotheses about associations between predictive factors and disease outcomes

Cross-Sectional StudiesLimitations:• Prevalent rather than incident (new)

cases are used – the exposure could be associated with survival after disease occurrence, rather than development of the disease

• Temporal sequence between exposure and disease cannot be established

* i.e. Which came first, chicken or the egg?

National Surveys

• A large set of national epidemiological surveys are conducted by the National Center for Health Statistics (NCHS) --- the Federal Government’s principal vital and health statistics agency.

• The NCHS is part of the Center for Disease Control and Prevention (CDC) – under the U.S. Dept. of Health and Human Services.

National Surveys

• National Health Interview Survey (NHIS)Principal source of information on the health of the civilian, non-institutionalized household population of the U.S.

• National Health and Nutrition Examination Survey (NHANES)Collects information about the health and diet of people in the U.S.

National Surveys

• National Health Care Survey (NHCS)Collects a wide range of data on health care use, impact of medical technology, and quality of care provided.

• National Immunization Survey (NIS)Collects information on the immunization coverage and health care of children across the U.S.

National Surveys

• National Survey of Family Growth (NSFG)Collects data on factors affecting pregnancy and women’s health in the U.S.

• National Mortality Followback Survey (NMFS)Aggregate sample of death certificates across the U.S. for targeted research purposes.

National Surveys• National Maternal and Infant Health

Survey (NMIHS)Collects data on study factors related to low birthweight, stillbirth, infant illness, and infant death.

• Behavioral Risk Factor Surveillance SystemCollects data on prevalence of personal health behaviors among adults associated with premature morbidity and mortality.

National Surveys/Programs

• Youth Risk Behavior Surveillance SystemCollects data to monitor prevalence of behaviors in youths that results in significant mortality, morbidity, disability, and social problems.

• National Program of Cancer Registries (NPCR)

• Surveillance, Epidemiology, and End Results (SEER) Program

Other Sources of National Data

• National Vital Statistics System--- Birth data--- Mortality data--- Fetal death data--- Marriages and divorces--- Linked births/infant deaths--- National death index

• U.S. Census Bureau