Question

What are the definitions and steps of prospective vs. retrospective design? ​

• Please give examples and comment on the pros and cons of each design

Answer 1

Prospective Cohort Studies:

In prospective cohort studies the investigators conceive and design the study, recruit subjects, and collect baseline exposure data on all subjects, before any of the subjects have developed any of the outcomes of interest. The subjects are then followed into the future in order to record the development of any of the outcomes of interest. The follow up can be conducted by mail questionnaires, by phone interviews, via the Internet, or in person with interviews, physical examinations, and laboratory or imaging tests. Combinations of these methods can also be used.

The illustration below shows a hypothetical group of 12 subjects followed over a number of years. They were enrolled into the study at different times, and some of them became lost to follow up, i.e., they stopped responding to letters, emails and phone calls, so we don't know what happened to them; these are show by the horizontal follow up line stopping.

Three subjects developed the outcome of interest at the approximate dates show by the "X"s. The incidence rate was calculated by computing the disease free observation time for each subject, adding up the disease-free observation times for the entire group, and then dividing this into the number of events, as shown in the calculation below the time line.

Retrospective Cohort Studies

Retrospective studies also group subjects based on their exposure status and compare their incidence of disease. However, in this case both exposure status and outcome are ascertained retrospectively.

When an outbreak of Giardia (see this Link to CDC page on Giardia) occurred in Milton, MA , the Milton Health Department requested assistance from the epidemiologists in the MA Department of Public Health. (Kathleen MacVarish from the BUSPH Practice Office was the Health Agent in Milton who led the investigation.) The request for assistance was made some time after the start of the outbreak, and the outbreak was winding down by the time DPH began their study. The outbreak was clearly concentrated among members of the Wollaston Golf Club in Milton, MA , which had two swimming pools, one for adults and a wading pool for infants and small children. Given what they knew about the usual mechanisms by which Giardia is transmitted, the investigators thought that contamination of the kiddy pool by a child shedding Giardia into their stool was the most likely source. (NOTE) The study was conducted by getting most of the people in the cohort to complete a questionnaire in which one of the key questions was "Did you spend any time in the kiddy pool?" This outbreak clearly took place in a well-defined cohort (members of the club), and the investigators could determine how many people developed Giardia in each of the exposure groups (i.e., exposed to the kiddy pool or not). Moreover, they also knew how many respondents had been exposed to the kiddy pool and how many were not. In other words, they knew the denominators for the exposure groups, so they could calculate the cumulative incidence, risk difference, and the risk ratio. They found that people who had spent time in the kiddy pool had 9.0 more cases per 100 persons than those who spent time in the kiddy pool. The risk ratio was 3.27. Because the investigation started after the cases had already occurred, DPH's study of Giardia in Milton is an example of a retrospective cohort study.