176 PART 4 Comparing Groups

(abbreviated HTN, outcome) over time. However, in a cross-sectional study,

exposure and outcome are measured at the same time, so you can only look for

associations. If your exposure and outcome are binary (such as obese: yes/no and

HTN: yes/no), you can use a fourfold table for this evaluation. But you would have

to develop a sampling strategy that would support your analytic plan.

The example given here is for the use of a fourfold table to interpret a cross-

sectional study. If you have heard of a fourfold table being analyzed as part of a

cohort study or longitudinal study, that is referring to a series of cross-sectional

studies done over time to the same group or cohort. Each round of data collection

is called a wave, and fourfold tables can be developed cross-sectionally (using data

from one wave), or longitudinally (using data from two waves).

As described in Chapter 6, you could try simple random sampling (SRS), but this

may not provide you with a balanced number of participants who are positive for

the exposure compared to negative to the exposure. If you are worried about this,

you could try stratified sampling on the exposure (such as requiring half the sam-

ple to be obese, and half the sample to not be obese). Although other sampling

strategies described in Chapter 6 could be used, SRS and stratified sampling are

the most common to use in cross-sectional study. Why is your sampling strategy

so important? As you see in the rest of this chapter, some indices are meaningful

only if the sampling is done accordingly so as to support a particular study design.

Producing Fourfold Tables in

a Variety of Situations

Fourfold tables can arise from a number of different scenarios, including the

following:»

» ^{Comparing proportions between two groups (see Chapter 12)»}

» ^{Testing whether two binary variables are associated»}

» ^{Assessing associations between exposures and outcomes»}

» ^{Evaluating diagnostic procedures»}

» ^{Evaluating therapies»}

» ^{Evaluating inter-rater reliability}

Note: These scenarios can also give rise to tables larger than 2

2, and fourfold

tables can arise in other scenarios besides these.