0:04

So now let's turn to the second of the major research designs,

Â I'm going to talk to you about, and that is cohort studies.

Â We're going to talk about cohort studies and case control studies.

Â It turns out if you're working in this field just starting out,

Â you're likely to do a case control study, before a cohort study.

Â But cohort study is conceptually prior, for

Â various reasons, so I'm putting that first.

Â 0:28

And here's the diagram,

Â this is the standard diagram.

Â A case control study and a cohort study differ, in their temporal orientation.

Â So a cohort study has a, a

Â putative exposure, something it thinks is dangerous or

Â risky, and it compares people exposed, to people

Â not exposed, and follows them forward in time.

Â And over time, some people acquire the disease, that would be the cases and

Â some people do not, that would be the controls.

Â And so without too much difficulty, we can make a rate of incidence, or prevalence.

Â We have, in the exposed group, a divided by a plus b, that would be the risk.

Â And we can compare that risk, to the risk in the not exposed, that

Â would be c, divided by C plus D, that would be the relative risk.

Â And we can compare those two and if they're different, we're

Â going to say, well that exposure is related to depressive disorder, let's say.

Â 1:31

It turns out, I'm showing you the relative odds also because

Â that turns out to be crucial, for the case control study.

Â You can see that, the quantity a over a plus b, is very similar

Â to the quantity, a over b. That depends actually, how big

Â a is, how many cases there are, and so we say with rare diseases, the odds ratio

Â a over b, is very similar to the risk ratio, a over a plus b.

Â So, if you're not a bookmaker, you're not used to

Â relative odds, but epidemiologists talk about relative odds all the time.

Â And we'll come back to this, when we talk about case control studies.

Â But the point is,

Â Cohort studies look forward in time and, as I'll

Â show you, case control studies look backward in time.

Â 2:21

Now, the exemplar case control study, that I've taken from

Â Lilienfeld and Stolley's book, is about smoking and lung cancer.

Â We're going to have a case control study and

Â a cohort study, about smoking and lung cancer.

Â This cohort study, they enlisted 68,000 families and

Â they got volunteers from the families, to inform about the people in the family.

Â And actually, it concerned a million people, in those 68,000 families.

Â And filled out questionnaires and they did annual follow ups.

Â They lost some people out of, there were 27.000

Â people that they didn't, were not able to follow for

Â one reason or other, but they got death certificates,

Â and there were 46,000 people that died over a three

Â year period, 1959 to 1962.

Â And, here's what it shows, I think you all know this.

Â Non-smokers have a lower rate of lung cancer.

Â And people who smoked 40 plus cigarettes per

Â day, had a very high rate of lung cancer.

Â And, one of the interesting things is, the so called dosage effect.

Â That is, you can see the more you smoke, the higher your risk for lung cancer.

Â So that is a prospective

Â study and that's pretty convincing, isn't it?

Â That lung cancer is related to smoking.

Â 3:37

So here's a cohort study for depressive disorder and what we

Â have to do is, we have to define the cohort event.

Â So in the cohort study of smoking, it was just the start of the study.

Â We had a population of about a million people.

Â Here, we went to find an exposure that we think might be risky.

Â And in this case, the exposure is, for women, a miscarriage.

Â 4:11

And we find that 25 of them, have an occurrence of major depressive disorder.

Â 25 over 229 is about 11%,

Â 10.9%.

Â and we have a community, controls, who are not exposed to miscarriage.

Â These are women, who are of similar age and education

Â and speak English, and they're matched on the season of

Â the year also, so there are 230 of them and

Â only 10 of them, have major depressive disorder, or 4.3%.

Â So you can now see, we're now thinking, a miscarriage may be

Â a risk factor for major depressive disorder and you can see, the relative

Â risk: 25 over 229, that's about 10.9% over 10, divided by 230.

Â That's about 4.3%, that generates a relative risk of 2.51.

Â Now you have to bear with me a little bit because

Â this, comes up again, in the case control study, the relative odds.

Â Let's look at the relative odds, which are not

Â 25 divided by 229, but 25 divided by 204.

Â Right,

Â 5:21

and again, the odds of depression in the "not exposed"

Â community group are 10 divided by not 230, but 220.

Â and you can see that, Relative Odds is very very similar to the Relative Risk.

Â Turns out to be crucial for the case control study, which comes up soon.

Â 5:42

So, the Cohort Study, the strengths of the Cohort Study

Â is, one strength is, its perspective.

Â So there's no, there's no, people can't, aren't

Â remembering things incorrectly because you're proceeding forward in time.

Â And you also can estimate incidence, which we discussed

Â earlier, as the closest approximation to the force of morbidity.

Â You can estimate incidence and because you

Â can estimate incidence, you can estimate relative risk.

Â But it turns out, the cohort study is hard to do, It's

Â expensive.

Â Often times in a cohort study, as in the smoking study, you have a

Â small percentage of people exposed and many,

Â many, many people who are not exposed.

Â But you have to measure them anyway, and that costs money.

Â And, if the disease takes a long time to occur, as in the

Â case of lung cancer, you have to follow them for a long time.

Â So that's a, a difficulty with

Â the cohort study.

Â And when you follow them for a long time, you will likely lose some

Â people, as happened in the cohort study of smoking, where we lost 27,000 people.

Â So, it could be that you'd lose the people, that

Â are most likely, or perhaps least likely, to acquire the disease.

Â And so we call that attrition bias.

Â So, those are strengths and weaknesses of the Cohort Study.

Â And now, let's go look at a different kind

Â of study, very similar, but the case control study.

Â