2.  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.

3. 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

4.  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.

5. • 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]

6.  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)

8.  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.

11.  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

12.  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

13.  The principle of this study design is
To identify changes in the frequency of an health event in
space and/or time
 To relate these changes with environmental factors

14.  The 2 major objectives of ecological
studies are:
4) to generate or to test etiologic hypotheses, in order to
explain disease occurrence;
5) to evaluate the impact of intervention programs or
policies (if we have the knowledge of disease etiology).

15.  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

16.  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)