Practical aspects of sharing controls between case-control studies

Purpose We discuss the practical advantages and challenges of sharing controls among two or more concurrently conducted case-control studies. Methods We conducted two case-control studies, one of breast cancer and the other of endometrial cancer, with overlapping, shared control groups. The studies had overlapping geographic areas, identical telephone questionnaires and biosample collection, and identical age and race eligibility. Results Sharing controls reduced the number of potential controls that had to be identified by random-digit dialing by 25% and the number of eligible controls that had to be interviewed by 32%. The cost savings were approximately $296 000, or 7% of the program project that funded the studies. Conclusions The disadvantage of sharing controls was the complexity of the design and the additional investigator time required to plan, monitor, and adjust the design. In the situation presented here, the complexities would have been reduced greatly if we had not attempted to frequency match on age in both studies. Generally, sharing controls is likely to work well when strict frequency matching is not required and there is a large overlap of interview questions, other data to be collected, and eligibility criteria among the studies. Copyright (c) 2005 John Wiley & Sons, Ltd.