Question

Why is cumulative incidence difficult to measure directly? Name one reason and explain why.

Answer 1

incidence is a measure of the occurrence of new cases of disease (or some other outcome) during a span ​ of​​​​​​ time. There are two related measures that are used in this regard: incidence proportion (cumulative incidence) and incidence rate. A useful way to think about cumulative incidence (incidence proportion) is that it is the probability ​​​​​​ of eveloping disease over ​​​​​ a tated period of time; as such, it is an estimate of risk.

the incidence proportion (cumulative incidence) must specify a time period. For example, the incidence proportion of neonatal mortality is the number of deaths divided by the number of births over the first 30 days after birth.This is particularly a problem when assessing long term risk.

A first problem is that, even if subjects don't die for another reason, it is difficult to follow people for long periods of time, and subjects can become lost to follow-up , which also means that their outcome status is unknown.
A second problem is that the incidence proportion doesn't distinguish when a disease occurs as long as it is within the follow-up period. For example, if a population is followed for 20 years, it would make a difference to the person and to the epidemiologist if the cancer occurred after two years or after 20 years, but both of these outcomes would count the same with the incidence proportion.

For this reason, the incidence proportion is generally used in situations where the follow-up time is relatively short and there is relatively little loss to follow-up