Clinically referred patients offer an alternative population, but have documented biases (1). If using such a population, careful consideration of data quality, and appropriate statistical analyses are required to increase the likelihood of valid and generalizable results.

References

(1) Hoover DR, Munoz A, Carey V, Odaka N, Taylor JM, Chmiel JS et al. The unseen sample in cohort studies: estimation of its size and effect. Multicenter AIDS Cohort Study. Stat Med 1991; 10(12):1993-2003.