In a clinic-based SS disease population, a patient arrives to clinic after disease onset. This phenomenon is known as �late-entry�, and is quantified by the period between birth and clinic arrival (1;2). Alternatively, an SS disease patient may never be recruited to the study, due to either early life mortality without clinic attendance, or mild disease expression not requiring clinical care.

References

(1) Simon R. Length biased sampling in etiologic studies. American Journal of Epidemiology 1980; 111(4):444-452.

(2) Brookmeyer R, Gail MH. Biases in prevalent cohorts. Biometrics 1987; 43(4):739-749.