he most direct way to determine if an exposure
is associated with a condition is to prospectively
follow 2 groups, one exposed and one unex
posed, and observe the frequency with which
each group develops the condition. Relative risk, dis
cussed in the first part of this 2part series on risk as
sessment (
Hospital Physician
, October 2005), reflects
the magnitude of the difference in the frequency of an
outcome between groups. For a given exposure (and
exposure could mean exposure to a risk factor or
exposure to a treatment), the relative risk of an out
come is calculated as:
Relative risk =
Risk of the outcome with exposure
Risk of the outcome without exposure
Consider a study of 1000 people in which 500 are
treated with a new drug and 500 are treated with place
bo. If 5 in the treatment group (1%) and 10 in the
placebo group (2%) have heart attacks, the relative
risk of heart attack with treatment would be 0.01/0.02,
or 0.5. The risk of heart attack with treatment is one
half the risk without treatment.
The relative risk tells us the precise proportional dif
ference in risk between groups, and, as we saw in the
previous article, can be easily calculated when groups
with known outcome frequencies are studied. There
are situations (eg, casecontrol studies) when all the
necessary information to calculate a relative risk may
not be available, but we still wish to measure the associ
ation between a risk factor and an outcome. Another
frequently presented measure of this association is the
odds ratio.
WHEN THERE IS NO DENOMINATOR
Calculating relative risk requires knowing the risk of
an outcome with and without exposure. To determine
risk, you must know not only the frequency of an out
come but also the size of the groups at risk. If you fol
lowed a defined group prospectively, you could precise
ly calculate the proportion that develops a condition of
interest. When you have a nearly “captive population”
(eg, those with a particular kind of insurance who
would almost certainly seek medical care in a particu
lar setting), you can make reasonable assumptions
about the size of the population, and thus make rea
sonable estimates of the true risk.
What if you are suspicious that a particular exposure
might increase the risk of developing a particular con
dition? Say you identify a group of patients with a can
cer and wish to determine if a particular exposure is a
risk factor. It would be impractical, particularly if you
are dealing with a rare condition, and unethical, to
expose diseasefree individuals to the potential risk fac
tor and measure the frequency of cancer development.
You could, however, review the records of those with
cancer and determine how many of them were ex
posed to the potential risk factor. You could then cre
ate a group of people otherwise similar, but without
cancer, and determine the proportion of this group
that was exposed to the potential risk factor. By per
forming this casecontrol study and comparing the
proportion of those exposed with cancer (the cases) to
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 Fall '09
 STACY
 Nutrition, Epidemiology

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