- [Voiceover] Hey guys, this is Dr. Joel with MedImmersion.
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in the Flash Flood Review Series,
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Okay, let's get started.
In this lecture, I'm going to be talking
about three different studies.
The case-control study, the cohort study,
and the cross-sectional study.
Alright, starting off with case-control studies,
and for this example, I'm going to use two groups of people.
The first group of people,
the red group of people, are the cases,
and those are the people that have a disease.
In this case, I'm gonna say it's skin cancer.
The people in the bottom are the second group,
the green people,
are those people that do not have the disease.
These are the controls.
Because of this naming,
you can see why it makes sense
to call this a case-control study.
So basically, this is a type of retrospective study,
meaning that we're looking backwards in time.
We already know who has cancer,
and who doesn't have cancer,
and we've already put them into groups.
So we're looking backwards in time,
retrospectively, to see why one group has a disease,
and one group doesn't.
So in this case,
we're comparing the subjects who have the disease,
the cases,
with those who do not have the disease, the controls.
And we're asking ourselves "What happened?
"What was different?
"Why does the first group have skin cancer,
"and the second group not have skin cancer?"
We can make a hypothesis, like,
we can say, well I think that the group with skin cancer
had excessive exposure to the sun when they were young,
and we look backwards in time,
and sure enough, we see that, yes,
they did have a higher exposure to the sun.
A case-control study is good for giving us the odds ratio,
which is the odds of getting a disease after exposure,
divided by odds of getting that
same disease without exposure.
So you need to understand that the top group in red,
not necessarily all of them did have
excessive sun exposure as a youth,
some of them just got cancer, just happened.
Likewise, the people in the green group,
some of them may have had
excessive sun exposure as a youth.
But really what we're looking at is the odds.
Are the odds increased to get cancer
when you have excessive sun exposure,
and the answer in this case is yes, your odds are higher.
Okay.
Now into the cohort study.
A cohort study is a type of prospective study.
Basically we're looking into the future now,
in which we take two groups of subjects,
with unknown outcomes.
And of course that makes sense,
because the future hasn't happened yet, right?
We're looking into the future,
we don't know what's going to happen.
But we're comparing these two groups based on
what we suppose is going to be a causal factor.
So you can see, there's two groups here.
We know that one of these groups has sun exposure,
and the other group doesn't.
We're asking ourselves "What will happen in the future?
"What is going to happen to these
"two groups of people in 20 years?"
A cohort study is good for measuring the relative risk,
which is the ratio of two probabilities.
The probability of a disease occurring in an exposed group,
divided by the probability of the disease
occurring in a non-exposed group.
So how is the relative risk different than the odds ratio?
Well, with the odds ratio,
we're comparing two groups of people with known disease.
For the relative risk,
we're comparing two groups with known exposure.
So having an exposure doesn't guarantee
that you're going to get the disease.
Likewise, having the disease doesn't
guarantee that you have the exposure.
So relative risk is kind of saying
what's the risk of getting the disease
based on having a lot of sun when you're a child,
and the odds ratio is what are the odds
that all that sun exposure is going to cause
a disease later on or contribute to a disease later on.
Okay.
Lastly, is the cross-sectional study.
This is a study that analyzes data from a
population of people or whatever you're studying,
at one point in time, right?
We're not looking forward in time,
we're not looking backward in time.
We're looking right now.
What's happening right now?
Thus it's across the current section of time,
or it's a cross-sectional data, and it asks the question,
not to be too repetitive, "What is happening?"
And cross-sectional studies are
good at measuring the disease prevalence,
which is the proportion of a population
found to have a condition, at a given point in time.
Alright guys, thanks for watching this video.
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Hang in there.