| Summary: | Abstract Background Participants in voluntary research present a different demographic profile than those who choose not to participate, affecting the generalizability of many studies. Efforts to evaluate these differences have faced challenges, as little information is available from non-participants. Leveraging data from a recent randomized controlled trial that used health administrative databases in a jurisdiction with universal medical coverage, we sought to compare the quality of care provided by participating and non-participating physicians prior to the program’s implementation in order to assess whether participating physicians provided a higher baseline quality of care. Methods We conducted clustered regression analyses of baseline data from provincial health administrative databases. Participants included all family physicians who were eligible to participate in the Improved Delivery of Cardiovascular Care (IDOCC) project, a quality improvement project rolled out in a geographically defined region in Ontario (Canada) between 2008 and 2011. We assessed 14 performance indicators representing measures of access, continuity, and recommended care for cancer screening and chronic disease management. Results In unadjusted and patient-adjusted models, patients of IDOCC-participating physicians had higher continuity scores at the provider (Odds Ratio (OR) [95% confidence interval]: 1.06 [1.03–1.09]) and practice (1.06 [1.04–1.08]) level, lower risk of emergency room visits (Rate Ratio (RR): 0.93 [0.88–0.97]) and hospitalizations (RR:0.87 [0.77–0.99]), and were more likely to have received recommended diabetes tests (OR: 1.25 [1.06–1.49]) and cancer screening for cervical cancer (OR: 1.32 [1.08–1.61] and breast cancer (OR: 1.32 [1.19–1.46]) than patients of non-participating physicians. Some indicators remained statistically significant in the model after adjusting for provider factors. Conclusions Our study demonstrated a participation bias for several quality indicators. Physician characteristics can explain some of these differences. Other underlying physician or practice attributes also influence interest in participating in quality improvement initiatives and existing quality levels. The standard for addressing participation bias by controlling for basic physician and practice level variables is inadequate for ensuring that results are generalizable to primary care providers and practices.
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