In single ascertainment a random small sample of all cases in the population are identified. Examples include identifying the affected children in a clinic or school grade. For single ascertainment the probability that a family will be identified increases with the number of affected offspring. The sib method is used to correct the segregation ratio for the bias of single ascertainment.


corrected segregation ratio by the sib method =

= ((total number of affected offspring) - (number of sibships)) / ((total number of offspring) - (number of sibships))


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