Referral sampling: Using physicians to recruit patients
Convenience sampling often is necessary when conducting research where there is no frame for a probability sample. This might include samples of people with rare diseases, persons recovering from compulsive behaviors, or those involved in some stigmatized activity. When there is no reliable list from which to draw a statistical sample, the best alternative may be to rely on sampling by referrals, sometimes called snowball sampling (Welch, 1975), respondent-driven sampling (Heckathorn, 2002), or multiplicity sampling (Rothbart, Fine & Sudman, 1982).
Hogan, S., Power, M., Schulkin, J., & Loft, J. (2009). Referral sampling: Using physicians to recruit patients. Survey Practice, December.