# “Have you ever …” as a question for measuring cumulative incidence

In Gordis Epidemiology, it is stated that the interview question Have you ever had asthma? can be used to measure cumulative incidence. Cumulative incidence was defined in the same book as: $$\frac{\text{number of new cases in the population}}{\text{number of people at risk in the population}}\text{over a specified time period}$$
My question is, how can the answer to Have you ever had asthma? measure this? Would the proportion of the 'yes's to the total number of people somehow be the cumulative incidence proportion, and if so, what would be the "time period" implicit in the calculation? How exactly is the 'new cases' in the numerator explained since this, in my opinion, looks more like prevalence than incidence?

• "New cases over a time period" helps measure cumulative incidence over that time period. "Have you ever?" helps measure cumulative incidence since birth. – Henry May 11 at 14:47

Cumulative incidence is not the same thing as the incidence proportion (i.e. one kind of incidence rate), unless you understand "period of time" to be a variable length period which always begins at birth. This distinction is made very clear if you recognize that the "over a specific period of time" in the definition of incidence proportion is something like a calendar year, decade, or month, and if you also recognize that the word "ever" in cumulative incidence makes a very different kind of period measure (one of variable length). Cumulative incidence becomes more interesting when restricted to specific groups such as 'all people born in a given year,' or 'all people in this treatment group'. As Gordis' used the term, cumulative incidence is the complement of the survivor function: $${1 - S(t)}^{\dagger}$$.
$${}^{\dagger}$$ That said, there are other uses of the term 'cumulative incidence' not appearing in Gordis which have different definitions.