as the result of a calculation from this fraction, but stated as a rate so that it is easy to envision. It

would be hard to envision that 0.0317 percent of influenza sufferers were hospitalized in one month.

On the other hand, it is much easier to envision almost 32 people from a town with a population of

100,000 being hospitalized in one month — provided you also envision that everyone in the town had

influenza.

Because prevalence is a proportion, it’s analyzed in exactly the same way as any other proportion. The

standard error (SE) of a prevalence ratio can be estimated by the formula in Chapter 13. Confidence

intervals (CIs) for a prevalence estimate can be obtained from exact methods based on the binomial

distribution or from formulas based on the normal approximation to the binomial distribution. Also,

prevalence can be compared between two or more populations using the chi-square or Fisher Exact

test. For this reason, the remainder of this chapter focuses on how to analyze incidence rates.

Incidence: Counting new cases

The incidence of a condition is the rate at which new cases of that condition appear in a

population. Incidence is generally expressed as an incidence rate (R), which — like prevalence

— is a fraction. The numerator for incidence is defined as the number of observed events (N) in a

particular time period. (Consider an event to mean that a member of the population goes from not

having the condition to having the condition.) Take note that while incidence expresses the

number of new cases of the condition in the numerator, in contrast, prevalence includes all cases

— both new and existing — in the numerator. The denominator for incidence is defined as the

number of individuals in the population who could have had the event multiplied by the interval

of time being used. This is also called time exposed or exposure (E). So, the equation for

incidence is the number of observed events divided by the exposure, which is (E): R = N/E.

Exposure is measured in units of person-time, such as person-days or person-years. Incidence rates are

expressed as the number of cases per unit of person-time. The unit of person-time is used so that the

incidence rate can at least be the size of a whole number so it is easier to interpret and compare.

The incidence rate should be estimated by counting events over a narrow enough interval of

time so that the number of observed events is a small fraction of the total population studied. One

year is narrow enough for calculating incidence of Type II diabetes in adults because 0.02 percent

of the adult population develops diabetes in a year. However, one year isn’t narrow enough to be

useful when considering the incidence of an acute condition like influenza. In influenza and other

infectious diseases, the intervals of interest would be in terms of daily, weekly, and monthly

trends. It’s not very helpful to know that 30 percent of the population came down with influenza in

a one-year period.

Consider City XYZ, which has a population of 300,000 adults. None of them has been diagnosed with

Type II diabetes. Suppose that in 2023, 30 adults from City XYZ were newly diagnosed with Type II

diabetes. The incidence of adult Type II diabetes in City XYZ would be calculated with a numerator of

30 cases and a denominator of 300,000 adults in one year. Using the incidence formula, this works out