An Evaluation of Different Strategies for Sampling Controls in an Online Case-Crossover Study of Acute Flares in Knee Osteoarthritis.

Our findings are sensitive to control sample selection. Under certain conditions, different patterns could be attributed to over reporting and social desirability bias, where people may want to present themselves more positively about their "usual" physical activity levels, at baseline. Exposure measurement at the time of a flare may be less precise and more susceptible to recall bias due to systematically reporting exposures differently during a flare, compared to control measurement independent of flares.