There are several diseases, including schizophrenia, alcoholism, and opiate addiction, for which the long-term prognosis is subject to disagreement between clinicians and researchers and also among researchers. Part of this disagreement may be attributable to a difference in the populations they sample. The clinician samples the population currently suffering from the disease (a "prevalence" or census sample), while research samples tend to more nearly represent the population ever contracting the disease (an "incidence" sample). The clinician's sample is biased toward cases of long duration, since the probability that a case will appear in a prevalence sample is proportional to its duration, hence "the clinician's illusion." The statistical mechanism of this bias is illustrated and its consequences detailed. Other sources of sampling bias in clinical and research samples are briefly described and partial remedies are suggested.