178 PART 4 Comparing Groups

Quantifying associations

How strongly is an exposure associated with an outcome? If you are considering

this question with respect to exposures and outcomes that are continuous vari-

ables, you would try to answer it with a scatter plot and start looking for correla-

tion and linear relationships, as discussed in Chapter 15. But in our case, with a

fourfold table, you are essentially asking: how strongly are the two levels of the

exposure represented in the rows associated with the two levels of the outcome

represented in the columns? In the case of a cohort study — where the exposure

is measured in participants without the outcome who are followed longitudinally

to see if they get the outcome — you can ask if the exposure was associated with

risk of the outcome or protection from the outcome. In a cohort design, you could

ask, “How much does being obese increase the likelihood of getting HTN?” You

can calculate two indices from the fourfold table that describe this increase, as you

discover in the following sections.

Relative risk and the risk ratio

In a cohort study, you seek to quantify the amount of risk (or probability) for the

outcome that is conferred by having the exposure. The risk of getting a negative

outcome is estimated as the fraction of participants who experience the outcome

during follow-up (because in a cohort design, all participants do not have the out-

come when they enter the study). Another term for risk is cumulative incidence rate

(CIR). You can calculate the CIR for the whole study, as well as separately for each

stratum of the exposure (in our case, obese and nonobese). Using the notation in

Figure 13-1, the CIR for participants with the exposure is a r

/ 1. For the example

from Figure 13-2, it’s 14 21

/

, which is 0.667 (66.7 percent). And for those without

the exposure, the CIR is represented by c r

/ 2. For this example, the CIR is calcu-

lated as 12 39

/

, which is 0.308 (30.8 percent).

The term exposure specifies a hypothesized cause of an outcome. If it is found that

a certain exposure typically causes risk for an outcome, it is called a risk factor, and

if it is found to confer protection, it is called a protective factor. Higher education

has been found to be a protective factor against many negative outcomes (such as

most injuries), and obesity has been found to be a risk factor for many negative

outcomes (such as HTN and Type II diabetes).

The term relative risk refers to the amount of risk one group has relative to another.

This chapter discusses different measures of relative risk that are to be used with

different study designs. It is important to acknowledge here that technically, the

term risk can only apply to cohort studies because you can only be at risk if you

possess the exposure but not the outcome for some period of time in a study, and

only cohort studies have this design feature. However, the other study designs —

including cross-sectional and case-control — intend to estimate the relative risk