Ecological Study

Data Collection

Ecological studies involve comparison and analysis of variables at the population level. They may involve direct observations of individuals which are then aggregated or summarized (to give means or proportions) or they may rely on global population measures, such as population density. Oftentimes, ecological studies rely on data previously collected for other purposes (e.g., population censuses and disease registries).

3. Define the minimal information you need to collect in order to test your ecological study hypothesis.

  1. I will need to know the size of the population of Epiville and the number of new cases of Susser Syndrome
  2. I will need to know the total number of new cases of Susser Syndrome and the size of the populations serviced by the Rothman and Greenland Reservoirs.
  3. I will need to know the geographic areas serviced by the Rothman and Greenland Reservoirs and the population size of those areas during a given time period. I will also need to know the number of new cases of Susser Syndrome as well as their place of residence.
Answer (a) — incorrect: While this information is important, more data is needed to test your specific hypothesis.
Answer (b) — incorrect: This information is necessary, but not sufficient to test your hypothesis
Answer (c) — correct: This information will allow you to cross-reference water use (via specific reservoir) and the Susser Syndrome cases

In order to get the information on the reservoirs, you contact the Department of Water Works. They report that all of Epiville is serviced by either the Rothman or Greenland Reservoir. The geographic areas served by each reservoir are subdivided into 5 sectors consistent with the Epiville residential subdivisions (Epiville Town Map). The Water Works Department has a database recording the average daily water use for each sector (but not for each individual living in the sector) for each of the previous 5 years. You decide that the information regarding the most recent year is adequate.

You also have access to the most recent Epiville population census and, because Susser Syndrome is a reportable disease, the Epiville Department of Health has reports of each diagnosed case along with the requisite demographic and residential information.

4. How can all of this information be used to test your study hypothesis?

  1. I can cross-reference the three databases in such a way that I will know from which reservoir sector the Susser syndrome cases originated, the population size of that sector, and the average daily water use of that population.
  2. I can cross-reference the three databases in such a way that I will know the population size of each sector as well as the average daily water use.
  3. I can use the Water Works Department information to extrapolate the amount of water each individual in Epiville consumed in the recent past and then cross-reference these individuals with the Susser Syndrome registry.
Answer (a) — correct: By cross-referencing the databases with address/geographic location, we can compare the differences in Susser Syndrome incidence by reservoir use
Answer (b) — incorrect: This information does not provide information on the actual incidence of Susser Syndrome per sector. Without these data, you cannot test your hypothesis.
Answer (c) — incorrect: In an ecological study we are interested in population-based statistics and not individual-based statistics. Extrapolating population-level information to the individual-level is known as ecological fallacy.

5. Which of the following is the primary shortcoming when using a reportable disease registry to collect Susser Syndrome cases in our study?

  1. We may not capture all of the Susser Syndrome cases occurring in Epiville.
  2. We would only capture prevalence data, not incidence data
  3. We are relying on the diagnostic capabilities of the doctors and hospitals and are assuming that all reported cases were valid.
Answer (a) — incorrect: Although a valid shortcoming in most studies, as we have defined it, all reportable diseases (including Susser Syndrome) must be reported to the Department of Health. Therefore, in a perfect world, no cases would go unreported (In fact, not all SARS cases were reported to the Department of Health through passive surveillance during the most recent outbreak).
Answer (b) — incorrect: The registry will have a date of initial diagnosis. As long as we have a clearly defined time component to our study, we will be able to capture incident as well as prevalent cases.
Answer (c) — correct: We do not have the time nor the resources to independently verify that all reported cases have been correctly diagnosed.

Dr. Zapp commends you on a job well done and instructs you on all of the administrative work that must be completed before you begin data analysis. Before you can begin you need to:

  1. Get approvals of the proposed study from database owners--this will ensure that the study adheres to the ethical principles of conducting public health research.
  2. Design a data management plan.
  3. Design a data analysis plan and propose how you will publicize your findings.