When all families with affected offspring are ascertained, we say that there is complete ascertainment; otherwise there is incomplete ascertainment and in this case (unknown to the investigator) there are families with affected siblings who are not probands. When there is complete ascertainment, the proband probability is one; otherwise it is distinctly less than one. Fisher [2] first analyzed the **data** using complete ascertainment. His analysis was done using a truncated Binomial distribution. However, Fisher [2] also described a simpler method for the more appropriate incomplete ascertainment for these data. This discussion was further developed by Bailey [7] and Morton [8]. In this paper, we will focus on incomplete ascertainment as is evident in Table  1. Crow [3] pointed out for the cystic fibrosis **data** the need to adjust for ascertainment bias and incomplete ascertainment.