Which of the following is true about cumulative incidence and incidence rate?

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Multiple Choice

Which of the following is true about cumulative incidence and incidence rate?

The main idea is distinguishing a risk over a defined period from a rate that uses person-time. Cumulative incidence is the proportion of people who are disease-free at the start and go on to develop the disease during a defined interval. It is calculated as the number of new cases during that period divided by the number at risk at the start (or by the person-time if you’re using a more precise approach). This makes it a measure of risk over the specified period—a proportion, not a rate. Incidence rate, in contrast, is a true rate that counts new cases per unit of person-time at risk. The denominator reflects the actual time each person contributes while at risk, so it handles varying follow-up times and censoring. Because of the person-time denominator, it expresses how fast new cases occur, not just what fraction eventually occur within a fixed window. So the statement that cumulative incidence is the proportion of initially disease-free individuals who develop disease during a defined period, and that incidence rate is the number of new cases per person-time at risk, captures the standard distinctions. Why the other ideas don’t fit: cumulative incidence isn’t a rate per person-time (that would be the incidence rate); it’s a proportion. It isn’t simply the number of all existing cases (that would be prevalence). And incidence rate isn’t the lifetime risk (lifetime risk is a cumulative incidence over a lifetime, not a rate).

The main idea is distinguishing a risk over a defined period from a rate that uses person-time. Cumulative incidence is the proportion of people who are disease-free at the start and go on to develop the disease during a defined interval. It is calculated as the number of new cases during that period divided by the number at risk at the start (or by the person-time if you’re using a more precise approach). This makes it a measure of risk over the specified period—a proportion, not a rate.

Incidence rate, in contrast, is a true rate that counts new cases per unit of person-time at risk. The denominator reflects the actual time each person contributes while at risk, so it handles varying follow-up times and censoring. Because of the person-time denominator, it expresses how fast new cases occur, not just what fraction eventually occur within a fixed window.

So the statement that cumulative incidence is the proportion of initially disease-free individuals who develop disease during a defined period, and that incidence rate is the number of new cases per person-time at risk, captures the standard distinctions.

Why the other ideas don’t fit: cumulative incidence isn’t a rate per person-time (that would be the incidence rate); it’s a proportion. It isn’t simply the number of all existing cases (that would be prevalence). And incidence rate isn’t the lifetime risk (lifetime risk is a cumulative incidence over a lifetime, not a rate).

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