Ecological Study

Study Design

The first step of your investigation is to generate a solid hypothesis. Once again, you look over the information that you have gathered regarding the Susser Syndrome cases and the water reservoirs in Epiville. Because you are designing an ecological study, you need data at the population level rather than the individual level.

1. Based on the facts presented, which of the following would be the most appropriate hypothesis to investigate using an ecological study?

  1. Residents of Epiville are at a higher risk of developing Susser Syndrome than residents of the neighboring towns.
  2. Employees of the Porks-A-Lot pig farm have a greater risk of developing Susser Syndrome than the general population of Epiville.
  3. The population served by the Rothman Reservoir has a higher incidence proportion of Susser Syndrome than does the population served by the Greenland Reservoir.
  4. Those diagnosed with Susser Syndrome will have greater odds of being a Porks-A-Lot employee than those without Susser Syndrome.
Answer (a) — incorrect: We do not have data for the neighboring towns to compare with that of Epiville. This hypothesis is too broad.
Answer (b) — incorrect: This hypothesis is more characteristic of a cohort study. We do not have the necessary data to test it
Answer (c) — correct: Ecological studies involve comparison and analysis at the population or group level. In our study, we have chosen to hypothesize that the Rothman Reservoir population will have a higher incidence proportion of Susser Syndrome than the Greenland Reservoir. (Note: The alternative would also be an appropriate hypothesis to test).
Answer (d) — incorrect: This hypothesis is more appropriate for a case-control study

2. Given that you are conducting an ecological study, define the source population from which you intend to draw the two comparison groups.

  1. Individual residents of Epiville who were recently diagnosed with Susser Syndrome
  2. All Epiville residents
  3. One study population should be all Susser Syndrome cases residing in Epiville; the comparison population should be healthy Epiville residents.
  4. The study population should be comprised of Epiville residents who are serviced by either the Rothman or Greenland Reservoirs.
Answer (a) — incorrect: Recall that we are undertaking an ecological study and are thus concerned with populations and not individuals.
Answer (b) — incorrect: This is not specific enough and only defines one study population. We are looking for two populations that we could compare.
Answer (c) — incorrect: These definitions are too specific and more appropriate for a case-control design.
Answer (d) — correct: This definition provides us with two study groups coming from the same source population. This will allow us to compare the incidence proportion of Susser Syndrome in the two areas using the two different sources of reservoir water.

With your hypothesis and source population defined, you now need to determine where you will get the necessary data. You don't have the time or resources to collect your own data, so you have to resort to using the pre-existing sources of information. Luckily, Epiville conducts routine data collections on almost everything--from the number of packs of cigarettes sold per capita to water and electricity consumption.