retrospective vs. prospective studies

Put simply, retrospective studies look back. Prospective studies look forward. But the differences go beyond that.

Flow chart diagrams of retrospective and prospective studies (Click for larger image)How many researchers does it take to change a light-bulb? None if it’s a retrospective study, because the light-bulb has already changed itself.

What this joke illustrates — besides the fact that good research jokes are hard to come by — is that studies generally fall into one of two categories: retrospective and prospective.

Let’s begin with a research question and see how it might be handled by each type of study. Say you want to know if physiotherapy improves return-to-work (RTW) outcomes among workers disabled by low-back pain.

Retrospective studies pose a question and look back. They use information that has usually been collected for reasons other than research, such as administrative data and medical records. Therefore, the outcome of interest has already occurred (or not) by the time the study is started.

In our example, researchers might turn to Workplace Safety and Insurance Board (WSIB) administrative data. They might retrieve low-back injury lost-time claims within a certain time frame, and collect information on medical treatments (physiotherapy) and return-to-work outcomes in order to look for associations among them.

Case-control studies are considered the highest quality of retrospective study because they try to approximate a control or comparison group. In our study, claim information would be collected on the population at risk: workers with low-back pain. They would be divided into two groups. The first group would be the control group, those who did not return to work. The second group would be the case group, those who did return to work. Claim information for workers who underwent physiotherapy might be compared to claim information on those who did not (the control group).

What the researchers would be looking for is an odds ratio: the odds of returning to work among those who received physiotherapy compared to the odds of returning among the no-physiotherapy group. An odds ratio of less than 1.0 would mean that RTW is less likely among those who received physiotherapy, and an odds ratio greater than 1.0 would mean that RTW is more likely among those who got physiotherapy treatment.

Despite this, retrospective studies are usually unable to reach cause-and-effect conclusions. For example, we cannot conclude that physiotherapy definitively improves RTW outcomes among back-injured workers. This is because of confounding factors — those unforeseen and unaccounted-for variables that may be affecting results. However, retrospective studies do give rise to hypotheses (e.g. that it looks like physiotherapy may improve RTW outcomes), which can then be further tested.

Prospective studies ask a question and look forward. The studies are designed before any information is collected. Study subjects are identified (workers with low- back injury claims) and followed forward to see if the outcome of interest (return to work) happens over time. This outcome is assessed relative to the intervention factor (physiotherapy).

Randomized controlled trials, considered the gold standard of study design, are prospective studies. They can provide evidence of cause-and-effect relationships and support changes in clinical practice or workplace interventions. In a randomized controlled trial, subjects are randomly assigned to receive the intervention or control treatment, and outcomes are evaluated after the intervention period. The control group is the group that receives standard care, no intervention or a placebo.

In our example, the researchers would randomly assign the workers with low-back injuries into two groups: one that is to receive physiotherapy and one that is not. These two groups would be followed over a period of time, and return-to-work outcomes among both would be noted.

The down side of prospective studies is that they are more expensive and time-consuming to design and carry out. As well, it is difficult to follow people for a long time, so situations in which there is a long wait between the exposure and outcome are not well suited to prospective studies. However, for reaching conclusions about the effectiveness of interventions, these studies are the most definitive.

Source: At Work, Issue 59, Winter 2010: Institute for Work & Health, Toronto

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