Behavioral Science

Epidemiology and Research Design 201
+
Actual disease state
–
+
a
b
Test result
–
c
d
Sensitivity = a/(a + c)
Specificity = d/(b + d )
PPV = a/(a + b)
NPV = d/(c + d)
a = True positives
b = False positives
c = False negatives
d = True negatives
All patients with disease = (a + c)
All patients without disease = (b + d )
All positive tests = (a + b)
All negative tests = (c + d )
Figure 26.1
How is sensitivity calculated?
How is specificity calculated?
How is positive predictive value
calculated?
How is negative predictive value
calculated?
True positives divided by all of the
individuals with a disease
Sensitivity = a/(a + c)
True negatives divided by all of the
people without a disease
Specificity = d/(b + d)
True positives divided by all people
with a positive test
PPV = a/(a + b)
True negatives divided by all people
with a negative test
NPV = d/(c + d)
RESEARCH STUDY DESIGNS
What is a prospective study?
A research study that starts before
the studied disease or outcome has
happened, and usually monitors for
its development.