cohort. So when we want to minimize the influence
of extraneous factors that may confound our
findings, we try to match our cases with our
controls in a case-control environment. If
you recall from our lecture on biases, confounding
was a classic problem we encounter a lot in
our studies and we like to stratify by that
variable that is the confounder to control
for. What does that mean? It means we match
or exclude or stratify analyses based upon
that variable. So matching is a process of
selecting the controls so that they are very
similar to the cases in certain characteristics.
Common characteristics tend to be of the classic
confounders; age, sex, race, socioeconomic
status, maybe occupation, maybe smoking status
and so forth. So let's say men are more likely
than women to develop lung cancer, that's
not necessarily the case, this is just an
example that's entirely hypothetical. Let's
also say that men are more likely than women
to have smoked. And we're going to run a case-control
study here. Sex is going to be a confounder
because gender is associated both of those
outcomes. By the way, I'm using the word sex
and gender interchangeably; they're technically
different constructs, so bear with me as I
fumble through those particular words.
One way to control for this is to match cases
and controls by sex. What does that mean?
It means that each case, each person with
lung cancer, is compared only to a specific
control in the other group of the same sex.
So men are compared to men and women are compared
to other women, and the twain shall never be
crossed. So the percentage of men across cases
will be equal to the percentage of men across
controls is another way of approaching it.
I'll say it again. I can either match individually
or through percentages. One is called individual
matching and the other is called group or
frequency matching. In individual matching,
I'm comparing one individual to another individual.
In group or frequency matching, I just make
sure that the numbers or proportions of the
individuals of interest are equal in my two
groups. Individual matching, group or frequency
So let's review what we've learned so far.
The cross-sectional study is when we ascertain
the exposure and the outcome status simultaneously.
It's great for surveys. It's great for measuring
associations between things that don't change,
typically, like gender, handedness, your height,
your eye color and so forth. The case-control study
is great in other contexts and the case-control
study is characterized by the fact that you
ascertain the outcome first, the lung cancer
and then you wait or look backwards to see
what the exposure status was, the smoking.
It's great when the outcome is rare, as in
rare diseases, or in outbreak investigations.
And the cohort study is when we ascertain
the exposure status first, we wait for time
to pass to see if the outcome manifests. It's
the default observational design. It's easy
to understand, we all love it, but it can