title: Simpson's paradox and experimental research.
abstract: BACKGROUND
  Experimental research in nursing has increased considerably in recent years. To improve the quality of such research, it is critical to reduce threats to internal validity. One threat that has received inadequate attention in the nursing literature is Simpson's paradox--a case of extreme confounding that can lead to erroneous conclusions about the effects of an experimental intervention. In fact, it can lead to a conclusion about an intervention effect that is the opposite of the correct inference.
  
  
  METHODS
  The aims of this study were to describe Simpson's paradox, provide a hypothetical example, and discuss approaches to avoiding the paradox.
  
  
  RESULTS
  The paradox is due to the combination of an overlooked confounding variable and a disproportionate allocation of that variable among experimental groups. Different designs and analysis approaches that can be used to avoid the paradox are presented.
  
  
  CONCLUSIONS
  Simpson's paradox can be avoided by selecting an appropriate experimental design and analysis that incorporates the confounding variable in such a way as to obtain unconfounded estimates of treatment effects, thus more accurately answering the research question.
authors.affiliation: Virginia Commonwealth University, 1100 East Leigh Street, Richmond, VA 23219, USA. swameringer@vcu.edu
authors.forename: Suzanne
authors.initials: S
authors.lastname: Ameringer
authors.forename: Ronald C
authors.initials: RC
authors.lastname: Serlin
authors.forename: Sandra
authors.initials: S
authors.lastname: Ward
mesh_terms: D000704:Analysis of Variance; D015982:Bias; D015986:Confounding Factors, Epidemiologic; D003627:Data Interpretation, Statistical; D015987:Effect Modifier, Epidemiologic; D006801:Humans; D015233:Models, Statistical; D015399:Nursing Research / Q000379:methods* / Q000592:standards; D016032:Randomized Controlled Trials as Topic; D012044:Regression Analysis; D015203:Reproducibility of Results; D012107:Research Design / Q000592:standards*; D012720:Severity of Illness Index; D016896:Treatment Outcome