Editing Sampling bias (section)

===Truncate selection in pedigree studies===
[[File:Ascertainment bias.png|600px|center|thumbnail|Simple pedigree example of sampling bias]]
Geneticists are limited in how they can obtain data from human populations. As an example, consider a human characteristic. We are interested in deciding if the characteristic is inherited as a [[autosomal recessive|simple Mendelian]] trait. Following the laws of [[Mendelian inheritance]], if the parents in a family do not have the characteristic, but carry the allele for it, they are carriers (e.g. a non-expressive [[heterozygote]]). In this case their children will each have a 25% chance of showing the characteristic. The problem arises because we can't tell which families have both parents as carriers (heterozygous) unless they have a child who exhibits the characteristic. The description follows the textbook by Sutton.<ref name="Sutton1988">{{cite book| vauthors = Sutton HE |title=An Introduction to Human Genetics|url=https://books.google.com/books?id=WY5qAAAAMAAJ|edition=4th|year=1988|publisher=Harcourt Brace Jovanovich|isbn=978-0-15-540099-3}}</ref>

The figure shows the pedigrees of all the possible families with two children when the parents are carriers (Aa).
* '''Nontruncate selection'''. In a perfect world we should be able to discover all such families with a gene including those who are simply carriers. In this situation the analysis would be free from ascertainment bias and the pedigrees would be under "nontruncate selection"  In practice, most studies identify, and include, families in a study based upon them having affected individuals.
* '''Truncate selection'''. When afflicted ''individuals'' have an equal chance of being included in a study this is called truncate selection, signifying the inadvertent exclusion (truncation) of families who are carriers for a gene. Because selection is performed on the individual level, families with two or more affected children would have a higher probability of becoming included in the study.
* '''Complete truncate selection''' is a special case where each ''family'' with an affected child has an equal chance of being selected for the study.

The probabilities of each of the families being selected is given in the figure, with the sample frequency of affected children also given. In this simple case, the researcher will look for a frequency of {{frac|4|7}} or {{frac|5|8}} for the characteristic, depending on the type of truncate selection used.