title: Matched designs and causal diagrams.
abstract: We use causal diagrams to illustrate the consequences of matching and the appropriate handling of matched variables in cohort and case-control studies. The matching process generally forces certain variables to be independent despite their being connected in the causal diagram, a phenomenon known as unfaithfulness. We show how causal diagrams can be used to visualize many previous results about matched studies. Cohort matching can prevent confounding by the matched variables, but censoring or other missing data and further adjustment may necessitate control of matching variables. Case-control matching generally does not prevent confounding by the matched variables, and control of matching variables may be necessary even if those were not confounders initially. Matching on variables that are affected by the exposure and the outcome, or intermediates between the exposure and the outcome, will ordinarily produce irremediable bias.
authors.affiliation: Department of Epidemiology and Biostatistics, School of Public Health, Tehran University of Medical Sciences, Tehran, Iran. mansournia_ma@yahoo.com
authors.forename: Mohammad A
authors.initials: MA
authors.lastname: Mansournia
authors.forename: Miguel A
authors.initials: MA
authors.lastname: Hernán
authors.forename: Sander
authors.initials: S
authors.lastname: Greenland
mesh_terms: D015982:Bias; D016022:Case-Control Studies; D015984:Causality; D015331:Cohort Studies; D015986:Confounding Factors, Epidemiologic; D006801:Humans; D016555:Matched-Pair Analysis; D012107:Research Design
keywords: Matching; bias; causal diagram; unfaithfulness
references.citation: Annu Rev Public Health. 1982;3:29-54
references.pmid: 6756431
references.citation: J Chronic Dis. 1983;36(10):685-97
references.pmid: 6630405
references.citation: Am J Epidemiol. 1990 Jan;131(1):151-9
references.pmid: 2293747
references.citation: Epidemiology. 2009 Jul;20(4):488-95
references.pmid: 19525685
references.citation: Int J Epidemiol. 2012 Oct;41(5):1480-9
references.pmid: 23045208
references.citation: Environ Health Perspect. 1994 Nov;102 Suppl 8:47-51
references.pmid: 7851330
references.citation: Epidemiology. 2009 Jul;20(4):512-22
references.pmid: 19487948
references.citation: Am J Epidemiol. 1986 Jul;124(1):150-3
references.pmid: 3717136
references.citation: Epidemiology. 1999 Jan;10(1):37-48
references.pmid: 9888278
references.citation: Epidemiology. 1996 Sep;7(5):498-501
references.pmid: 8862980
references.citation: Epidemiology. 2004 Sep;15(5):615-25
references.pmid: 15308962
references.citation: J Natl Cancer Inst. 1959 Jan;22(1):173-203
references.pmid: 13621204
references.citation: Epidemiology. 2003 May;14(3):300-6
references.pmid: 12859030
references.citation: Am J Epidemiol. 1982 Sep;116(3):547-53
references.pmid: 7124721