Rationale and Ecological Variables Section
An ecological study is an observational study in which at least one variable is measured at the group level. An ecological study is especially appropriate for the initial investigation of causal hypotheses.
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 of ecological variables, for which there is no correlation at the individual level
- We have limited funds or limited time to do the study
Three types of ecological variables:
- Aggregate Variables
- A summary or composite measure derived from values collected from individual members of a 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 that the content of minerals in drinking water is 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 analog 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 outcomes. 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.
Advantages & Disadvantages of Ecological Studies
- Can be done quickly and inexpensively bc rely on pre-existing data
- Analysis and presentation are relatively simple
- Can achieve a wider range of exposure levels than can be expected from an individual-level study
- Help explain population-level associations
- Ecological fallacy - the possibility of making incorrect conclusions about individual-level associations when only using group-level data
- Lack of information on important variables
Analysis of Ecologic Studies
Analytic models in ecologic studies are of different forms:
- Completely Ecologic
- All variables (outcome, exposure, and covariates) are ecological.
- Partially Ecologic
- Some, but not all, variables are ecological.
- Analyses may simultaneously include individual and ecological variables on the same construct (e.g., income). This could be called multilevel modeling, hierarchical regression, or mixed-effects modeling.