Ecological study


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Ecological study

  1. 1.  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.
  2. 2. 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
  3. 3.  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.
  4. 4. • 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]
  5. 5.  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)
  6. 6.  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.
  7. 7.  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
  8. 8.  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
  9. 9.  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
  10. 10.  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).
  11. 11.  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
  12. 12.  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)