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Overmatching is matching for an apparent mediator that actually is a result of the exposure. If the mediator itself is stratified, an obscured relation of the exposure to the disease would highly be likely to be induced.

Overmatching is matching for an apparent mediator that actually is a result of the exposure. If the mediator itself is stratified, an obscured relation of the exposure to the disease would highly be likely to be induced.

 

For example, matching the control group by gestation length and/or the number of [[multiple birth]]s when estimating [[perinatal mortality]] and birthweight after [[in vitro fertilization]] (IVF) is overmatching, since IVF itself increases the risk of premature birth and multiple birth.<ref>{{cite journal |title=The danger of overmatching in studies of the perinatal mortality and birthweight of infants born after assisted conception |last1=Gissler |first1=M. |last2=Hemminki |first2=E. |journal=Eur J Obstet Gynecol Reprod Biol |year=1996 |volume=69 |issue=2 |pages=73–75 |pmid=8902436 |doi=10.1016/0301-2115(95)02517-0}}</ref>

== Overmatching ==

For example, matching the control group by gestation length and/or the number of multiple births when estimating perinatal mortality and birthweight after in vitro fertilization (IVF) is overmatching, since IVF itself increases the risk of premature birth and multiple birth.

For example, matching the control group by gestation length and/or the number of multiple births when estimating perinatal mortality and birthweight after in vitro fertilization (IVF) is overmatching, since IVF itself increases the risk of premature birth and multiple birth.

''Overmatching'' is matching for an apparent mediator that actually is a result of the exposure. If the mediator itself is stratified, an obscured relation of the exposure to the disease would highly be likely to be induced.<ref name=marsh/> Overmatching thus causes [[statistical bias]].<ref name=marsh>{{cite journal |title=Removal of radiation dose response effects: an example of over-matching |last1=Marsh |first1=J. L. |last2=Hutton |first2=J. L. | author2-link = Jane Hutton |last3=Binks |first3=K. |year=2002 |journal=[[British Medical Journal]] |volume=325 |issue=7359 |pages=327–330 |pmid=12169512 |doi=10.1136/bmj.325.7359.327 |pmc=1123834}}</ref>

It may be regarded as a [[sampling bias]] in decreasing the [[external validity]] of a study, because the controls become more similar to the cases in regard to exposure than the general population.

It may be regarded as a sampling bias in decreasing the external validity of a study, because the controls become more similar to the cases in regard to exposure than the general population.

It may be regarded as a sampling bias in decreasing the external validity of a study, because the controls become more similar to the cases in regard to exposure than the general population.