Nurs Res. 2009 Mar-Apr;58(2):123-7

Ameringer S, Serlin RC, Ward S. Virginia Commonwealth University, 1100 East Leigh Street, Richmond, VA 23219, USA

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.