The incidence rate in unscreened women can be observed, as in the control group of a randomised trial, or it can be estimated on the basis of the incidence before screening or in other populations not offered screening 11 .
The incidence rate in unscreened women can be observed, as in the control group of a randomised trial, or it can be estimated on the basis of the incidence before screening or in other populations not offered screening 10 .