194 PART 4 Comparing Groups

incidence rate for City XYZ, which is 10 new cases for 100,000 person-years.

(Looks like City ABC’s public health department needs to get advice from City

XYZ!)

Understanding how incidence and

prevalence are related

From the definitions and examples in the preceding sections, you see that inci-

dence and prevalence are two related but distinct concepts. The incidence rate

tells you how fast new cases of some condition arise in a population, and preva-

lence tells you what fraction of the population has that condition at any moment.

You may expect that conditions with higher incidence rates would have higher

prevalence than conditions with lower incidence rates. This is true with common

chronic conditions, such as hypertension. But if a condition is acute — including

infectious diseases, such as influenza and COVID-19 — the duration of the condi-

tion may be short. In such a scenario, a high incidence rate may not be paired with

a high prevalence. Relatively rare chronic diseases of long duration — such as

dementia — have low yearly incidence rates, but as human health improves and

humans live longer on average, the prevalence of dementia increases.

Analyzing Incidence Rates

The preceding sections show you how to calculate incidence rates and express

them in larger units that are easier to envision. But, as we emphasize in Chapter 10,

whenever you report an estimate you’ve calculated, you should also indicate the

level of precision of that estimate. How precise are those incident rates? And

how can you tell when the difference between two incidence rates is statistically

significant? The next sections show you how to calculate standard errors (SEs)

and confidence intervals (CIs) for incidence rates, and how to compare incidence

rates between two populations.

Expressing the precision of an

incidence rate

The precision of an incidence rate (R) is expressed using a confidence interval (CI).

The SE of R typically is not reported, because the event rate usually isn’t normally

distributed. The SE is computed only as part of the CI calculation.