The challenge of conducting comparative effectiveness research in cancer: the impact of a fragmented U.S. health-care system.

Comparative effectiveness research (CER) can make important contributions to the transformation of US health care by filling gaps left by tightly controlled clinical trials. However, without comprehensive and comparable data that reflect the diversity of the US health-care system, CER's value will be diminished. We document the limits of observational CER by examining the age at diagnosis, disease stage, and select measures of health-care use among individuals diagnosed with incident cancer aged 65 or older from four large health maintenance organizations (HMOs) relative to seniors identified through the linked Surveillance, Epidemiology, and End Results (SEER)-Medicare data for the period 1999-2007. Aged individuals in the HMOs were younger, diagnosed at earlier stages, and more likely to receive care in inpatient settings than individuals in the linked SEER-Medicare data. These differences highlight the need for comprehensive and comparable datasets that reflect the diversity of US health care to support CER that can inform health-care reform in the United States.