CHAPTER 13 Taking a Closer Look at Fourfold Tables 187

Positive predictive value and negative

predictive value

The positive predictive value (PPV, also called predictive value positive) is the frac-

tion of all positive test results that are true positives. In the case of the pregnancy

test scenario, the PPV would be the fraction of the time a positive screening test

result means that the woman is truly pregnant. PPV is the likelihood that a posi-

tive test result is correct. You calculate PPV as a r

/ 1. For the data in Figure 13-4,

the PPV is 33 45

/

, which is 0.73. So, if the pregnancy test result is positive, there’s

a 73 percent chance that the woman is truly pregnant.

The negative predictive value (NPV, also called predictive value negative) is the

fraction of all negative test results that are true negatives. In the case of the preg-

nancy test scenario, the NPV is the fraction of the time a negative screening test

results means the woman is truly not pregnant. NPV is the likelihood a negative

test result is correct. You calculate NPV as d

r

/ 2. For the data in Figure 13-4, the

NPV is 51 55

/

, which is 0.93. So, if the pregnancy test result is negative, there’s a

93 percent chance that the woman is truly not pregnant.

Investigating treatments

In conditions where there are no known treatments, one of the simplest ways to

investigate a new treatment (such as a drug or surgical procedure) is to compare

it to a placebo or sham condition using a clinical trial study design. Because many

forms of dementia have no known treatment, it would be ethical to compare new

treatments for dementia to placebo or a sham treatment in a clinical trial. In those

cases, patients with the condition under study would be randomized (randomly

assigned) to an active group (taking the real treatment) and a control group (that

would receive the sham treatment), as randomization is a required feature of clin-

ical trials. Because some of the participants in the control group may appear to

improve, it is important that participants are blinded as to their group assign-

ment, so that you can tell if outcomes are actually improved in the treatment

compared to the control group.

Suppose you conduct a study where you enroll 200 patients with mild dementia

symptoms, then randomize them so that 100 receive an experimental drug

intended for mild dementia symptoms, and 100 receive a placebo. You have the

participants take their assigned product for six weeks, then you record whether

each participant felt that the product helped their dementia symptoms. You tabu-

late the results in a fourfold table, like Figure 13-5.