Unstratified Cohort Studies with a Dichotomous Disease Outcome
Ref:
The terms "case-control" and "cohort" studies are sometimes used interchangeably to describe studies which are designed to identify the relationship between the exposure to a risk factor and the subsequent development of a particular disease. However, there is an important distinction between the two studies and the sample size requirements for these studies are different.
The cohort study may be considered as a prospective or follow-up study. This study uses two groups of individuals. The "cases" comprise that group which has been "exposed" to some factor thought to be associated with the probability, p1, of the subsequent occurrence of some disease or event. These could be the "treated" group in a randomized trial where an event is observed or not in a prespecified time interval.
The "controls" are those individuals who have not been exposed to the suspected risk factor but still have some probability, p0, of developing the disease or event. These could be the comparison group in the previously mentioned trial. Apart from the specification of the levels of the Type I (alpha) and Type II (beta) errors, the sample size requirements depend on the incidence of the event among the non-exposed p0, and the relative odds (odds ratio), R, of disease in the exposed which one regards as important to detect: R = [p1 (1-p0)/p0 (1-p1)]
The program follows the development of calculations in Sections 6.2 & 6.3 in Schlesselman's book for the unmatched situation. In particular, equations 6.6, 6.7, 6.9 and 6.11 are used. For the matched case-control study, Section 6.6 is followed using equations 6.20, 6.22, 6.23 and 6.24.
The user must specify either the probability of disease incidence in the non-exposed controls for the unmatched situation. The program can compute any one of the following:
when the user supplies values for the other three.