Editing Selection bias (section)

===Attrition===
''Attrition bias'' is a kind of selection bias caused by attrition (loss of participants),<ref name=Juni>{{Cite journal | doi = 10.1093/ije/dyh406| title = Empirical evidence of attrition bias in clinical trials| journal = International Journal of Epidemiology| volume = 34 |issue=1| pages = 87–88| year = 2005| last1 = Jüni | first1 = P. |last2=Egger |first2=Matthias | pmid = 15649954| doi-access = free}}</ref> discounting trial subjects/tests that did not run to completion. It is closely related to the [[survivorship bias]], where only the subjects that "survived" a process are included in the analysis or the [[failure bias]], where only the subjects that "failed" a process are included. It includes ''dropout'', ''nonresponse'' (lower [[response rate (survey)|response rate]]), ''withdrawal'' and ''protocol deviators''. It gives biased results where it is unequal in regard to exposure and/or outcome. For example, in a test of a dieting program, the researcher may simply reject everyone who drops out of the trial, but most of those who drop out are those for whom it was not working. Different loss of subjects in intervention and comparison group may change the characteristics of these groups and outcomes irrespective of the studied [[health intervention|intervention]].<ref name=Juni/>

[[Lost to follow-up]], is another form of Attrition bias, mainly occurring in medicinal studies over a lengthy time period. Non-Response or Retention bias can be influenced by a number of both tangible and intangible factors, such as; wealth, education, altruism, initial understanding of the study and its requirements.<ref name="Jordan">{{Cite journal|last1=Jordan|first1=Sue|last2=Watkins|first2=Alan|last3=Storey|first3=Mel|last4=Allen|first4=Steven J.|last5=Brooks|first5=Caroline J.|last6=Garaiova|first6=Iveta|last7=Heaven|first7=Martin L.|last8=Jones|first8=Ruth|last9=Plummer|first9=Sue F.|last10=Russell|first10=Ian T.|last11=Thornton|first11=Catherine A.|date=2013-07-09|title=Volunteer Bias in Recruitment, Retention, and Blood Sample Donation in a Randomised Controlled Trial Involving Mothers and Their Children at Six Months and Two Years: A Longitudinal Analysis|journal=PLOS ONE|volume=8|issue=7|pages=e67912|doi=10.1371/journal.pone.0067912|issn=1932-6203|pmc=3706448|pmid=23874465|bibcode=2013PLoSO...867912J|doi-access=free}}</ref> Researchers may also be incapable of conducting follow-up contact resulting from inadequate identifying information and contact details collected during the initial recruitment and research phase.<ref>{{Cite journal|last=Small|first=W. P.|title=Lost to Follow-Up|date=1967-05-06|url=http://www.sciencedirect.com/science/article/pii/S014067366792377X|journal=The Lancet|series=Originally published as Volume 1, Issue 7497|language=en|volume=289|issue=7497|pages=997–999|doi=10.1016/S0140-6736(67)92377-X|pmid=4164620|s2cid=27683727 |issn=0140-6736|url-access=subscription}}</ref>