00:01
So in order to
do this, we set up what's called a contingency
table. We call it a contingency table because
the outcome is contingent upon whether or
not you get the exposure. Now pay close attention
because the way you set up this table will
determine whether or not the formulas that
I show you will actually work. It is important
to do it this way, where the exposure status
is on the horizontal levels of the table and
the outcome status is the vertical components
of the table. Some textbooks will transpose
that information as a way of testing you.
00:32
So if you're referring to a textbook, make
sure that they set up their tables the same
way that we're doing it here, otherwise the
formulas won't work. As well, the positive
must be the first thing, for example, whether
or not you get the exposure, yes is first,
no is second. Yes outcome is first, no outcome
is second. It's important that this contingency
table looks like your contingency table, otherwise
the formulas will not work. So we're going
to populate this contingency table with information.
For people who smoke and get lung cancer or
had a risk factor and get the outcome, we
call that A. If you have the exposure and
not get the outcome, let's say you're a smoker
who doesn't get lung cancer, you're in category
B. If you don't smoke and you still get lung
cancer, you're category C. And if you don't
smoke and you don't get lung cancer, we assume
this is the majority of people, that's outcome
or category D. Now we have a total number
of people in our set, that would be N, we
call the sums on the Ns of our contingency
table, marginals. So some important concepts
to consider, the first is absolute risk. When
we talk about risk, we're talking about incidence.
01:45
You may recall that incidence is a proportion
of new cases that we encounter, if I start
smoking and over some time, I get lung cancer,
that my experience represents a single incident
case, because it's new. If I had cancer before
I started smoking, it's not incident because
it is already there. So the absolute risk
is, if you're exposed to a factor, what's
the risk that you'll get that outcome. If
I'm a smoker, what's the risk that I will
get lung cancer, that's the absolute risk.
The relative risk on the other hand, is when
I compare the absolute risk of getting the
outcome if I'm a smoker, to the absolute risk
of getting the outcome if I'm not a smoker.
To put them in general terms, the risk of
the outcome when you're exposed versus the
risk of the outcome when you're not exposed,
that's relative risk. We are also going to
explore, in another lecture, attributable
risk, that's when we try to compute how much
of my outcome was actually due to the behavior
that I care about, how much of the lung cancer
was actually due to the smoking and not to
other factors. So absolute risk again is a
proportion of people with a yes outcome. There
are two kinds of absolute risk, there is the
absolute risk amongst those who did the behavior,
in other words, those who were exposed to
the exposure and again go back to our contingency
table, we give that as a over a + b, the number
of people who got the outcome who smoked,
divided by all the people who smoked. As opposed
to the absolute risk in the unexposed group,
that's the number people who got the outcome,
in this case lung cancer, divided by everybody
who didn't smoke. Two concepts there, absolute
risk in the exposed group, absolute risk in
the unexposed group. Now I am summarizing
it one more time, the cumulative incidence
rate amongst the exposed is a over a + b.
Again, that's the risk of getting the outcome
if you smoked or had the behavior, absolute
risk in the exposed group. The cumulative
incidence rate in the unexposed group is the
other side of the argument, c over c + d.
03:57
In other words, the risk of getting the outcome,
lung cancer, if you didn't smoke.
04:06
So back to our table again, you see where
those numbers come from. Now let us talk about
the absolute risk reduction, that's how much
my chances of having the outcome changed by
not doing the exposure, my chances of getting
lung cancer, how much did that reduce or increase
when I didn't smoke. So again, the cumulative
incidence rate amongst the exposed group,
a over a + b, again, that's my absolute risk
in my exposed group. The absolute risk in
the unexposed group once again, c over c + d.
The difference between the two, that's
my absolute risk reduction, how much risk
I had, from being exposed to smoking, minus
how much risk I had when I didn't smoke, that
difference tells me a lot. It tells me how
much I have gained in terms of health by not
doing that behavior.