So...why conduct an ecological study? Several reasons support using an ecological study design.
- The hypothesis is relatively new
- Adequate measurement of individual-level variables is not possible
- Adequate design of an individual-level study is not possible (i.e., not ethical)
- We are interested in the effect from ecological variables, for which there is no correlate at the individual level
- We have limited funds or limited time to do the study
Causal inferences from ecological studies can be made at three different levels.
- Biological. Causation is only through a biological pathway.
- Among unvaccinated persons, low-income people have increased risk of Hepatitis B because having a low-income increases a person's stress which increases a person's risk for Hepatitis B.
- Ecological. Causation is only through a group characteristic.
- Among unvaccinated persons, people living in a low-income community have increased risk for Hepatitis B infection because low-income communities have high levels of exposure to Hepatitis B.
- Contextual. Causation is through both biologic and group characteristics.
- Among unvaccinated persons, both high-income and low-income people who live in a low-income community have increased risk for Hepatitis B infection because of increased stress and increased exposure.
There are three types of ecological variables:
- Aggregate Variables
- A summary or composite measure derived from values collected from individuals. Examples would be the mean blood pressure or the rate of disease. In these cases, blood pressure is recorded for each individual in the study but the data are collapsed to a mean summary meausure for all people in the study. Similarly, only individuals get a cancer, but the rate of cancer is a summary measure for all the individuals in that population. Aggregate variables can measure exposure (e.g., mean blood pressure) or outcome (e.g., rate of disease) variables. One limitation with aggregate measures is that there is variation within the population - not all the individuals in the population have the average blood pressure.
- Environmental Variables
- A measure of the physical characteristics of the environment in which people reside, work, recreate or attend school. For example, we might hypothesize that rainfall is a risk for a fungal disease or the content of minerals in drinking water are protective against a certain disease. Therefore, environmental variables would be the mean rainfall in a geographic area or the mean level of minerals in drinking water. Environmental variables measure exposure, not outcomes. One limitation of an environmental variable is that there is variation in exposure levels for individuals in the population.
- Global Variables (Measure Exposure)
- A measure of the attributes of groups, organizations, or places for which there is no analogue at the individual level. For example, the procedures or treatments that are covered in a health insurance plan might affect the rate of disease or adverse health outcome. Additionally, population density would be another global variable because crowding might be an important exposure. There is no individual population density! Global variables are used to measure exposures, not outcomes.