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Prospective and Retrospective Studies

Prospective and Retrospective Studies

Author: Ryan Backman

Differentiate between prospective and retrospective studies.

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Hi, this is the tutorial on observational studies. So what we're going to start with is two research questions. Question 1-- are women more likely than men to support democratic candidates? And question 2-- does a new interactive computer program improve the reading ability of kindergarten students?

So if we're going to go about answering these two research questions, we would need some data. So the question is now is, how do we collect data to help answer each question? So let's just go back and think of a way of collecting data for each question. So if we looked at question 1, to answer this question, we would need to start with two samples, a sample of men and a sample of women.

And we would start with the sample of women and just simply ask each participant, do you support democratic candidates? You'd just record yes or no, and then you would go to the men, and you'd ask him the same question. Do you support democratic candidates, and then again select yes or no.

We would need to then figure out, well, what proportion of women support democratic candidates? What proportion of men support democratic candidates? And whichever one has the higher proportion, we could then to answer this question. So if men are higher than women, then, yes, they are more likely to support democratic candidates than men. So that would give us a good way of answering question 1. Question 2, we're going to have to collect data and using a different, really, a different means of statistical study.

So what we're going to need to do is go in to a kindergarten classroom. And the first thing I would do is give all of the kindergarten students a pre-test. So we would do a pre-test on their reading skills. After the pre-test, we would somehow randomly put them into two different groups. One group would represent the-- would be taught reading in just in the traditional manner that the schools always use to teach reading. And the second group would get their reading instruction via this interactive computer program. So then, after maybe a couple weeks of instruction, we would then test all of the students again.

We would then need to see, well-- so then we would test the traditional instruction group and then also the computer program group. Then we would take the pre-test , scores compare them to the post-test scores for each group. Then, whichever group had bigger gains in reading would be the group that did better. So if the computer program group did better than the traditional group, then we could say, yes, it does seem like this computer program improves the reading ability. So both of these two statistical studies are pretty different, but only one of them is what we call an observational study.

And what an observational study is a study that observes individuals and measures variables of interest, but does not attempt to influence the response. OK, so in this case, the response would be the reading ability. So now, based on that definition, which of the two research questions could be answered using an observational study, question 1 or question 2? And the answer to that is question 1.

So it would be question 1 because we're just simply trying to determine, just observe people's belief, people's political inclinations, political beliefs. We're not trying to influence whether they want to vote democrat or not. We're just simply observing what's true. Whereas, in that second research question, we are trying to influence the response. We're trying to see if students read better using this computer program. So question 2 would definitely not be an observational study, because we are trying to influence the response there.

But now, because of that, an observational study is a poor way to gauge the effect of an intervention. So it's a poor way to gauge an effect. So when our goal is to understand a cause-and-effect relationship, we cannot conduct an observational study. We can observe what's happening, but we can't definitively conclude that there is a cause-and-effect relationship.

So when we are conducting our observational study, to answer question 1, the people that were surveyed about their political inclinations were called the participants in the study. Whereas, animals are things that are part of an observational study are generally called subjects. So if they're people, they're participants. Really, anything else, we call them subjects.

Now, there are really two common types of observational studies. One's called a retrospective study, the prefix they're being retro. And we also have a prospective study where the prefix here is pro. Sometimes you'll hear a prospective study called a longitudinal study. So a retrospective study is a study that looks backwards and examines exposures to suspected risk or protection factors in relation to an outcome that is established at the start of a study.

A very common-- a very famous retrospective study was done in the early 1900s involving the link between cigarette smoking and lung cancer. So basically, what they did is they looked at two different groups. They took a group of smokers and a group of nonsmokers. And basically, they just, from each sample, determined what was the incidence of lung cancer in each of those groups.

And they were able to conclude that there is a higher incidence of lung cancer in the smoker group. So in many retrospective medical studies, a case control study is used to compare an exposed group to an unexposed group. So in historical example, the exposed group was the smokers. The unexposed group was the nonsmokers.

Now, a prospective study is a study that watches for outcomes, such as the development of a disease during the study period and relates this to other factors, such as suspected risk or protection factors. The study, also called a longitudinal study, usually involves taking a cohort of subjects and watching them over a long period of time.

So perhaps if a researcher is interested in looking at exposure to something like asbestos, maybe they would go into a factory and recruit a group, a cohort, of subjects that work in that factory and then do tests on them to see what their exposure to asbestos might be. So that's an example of where a prospective study might be used.

All right, well, that is your tutorial on observational studies. Thanks for watching.

Terms to Know
Observational Study

A type of study where researchers can observe the participants, but not affect the behavior or outcomes in any way.

Prospective Study

A study that begins by selecting participants, then tracks them and keeps data on the subjects as they go into the future.

Retrospective Study

A study that observes what happened to the subjects in the past, in an effort to understand how they became the way they are in the present.


The people or things being examined in an observational study.