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• 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.

• We explored a causal sequence between panic and avoidance to provide recommendations for psychotherapy, pharmacotherapy, and their combination in treating agoraphobia. We produced a two-way ([imipramine hydrochloride vs placebo] by [office-based behavioral therapy vs in vivo exposure]) design by amalgamating two studies. We assessed agoraphobic patients for panic and avoidance at these time points: baseline (week 0), midcourse (week 13), and termination (week 26). The causal sequence model was tested by path analysis. Imipramine was superior to placebo in lowering panic and avoidance at both postbaseline time points. Exposure was superior to office-based treatment in lowering avoidance only at week 13. Exposure appeared to produce quicker improvement of avoidance than office-based therapy, but relapse occurred if this improvement was not supported by medication. Exposure did not benefit panic. We believe patients should be informed that imipramine is superior to exposure in inducing a panic-free state. Exposure without imipramine is of benefit only in reducing avoidance, but adding imipramine to exposure is necessary for panic control and substantially improves exposure and exposure maintenance.

• This study examines the symptom checklist 90 (SCL-90), a patient self-report rating scale, as a screening and baseline psychopathology measure in an outpatient clinic. A low correlation (r=.17) was found between the SCL-90 and the SCL analogue, a matched psychopathology instrument rated by clinicians. The most common causes of discordance in ratings were patients' underreporting of symptoms due to patients' paranoia or fearfulness, overreporting of symptoms consistent with patients' "demonstrative" style, and false-positive Items related to patients' physical illness. We conclude that the SCL-90 has limited validity as a clinical measure in the study of patients seen in evaluation in our setting.

IN REPLY.— Dr Derogatis called the SCL-901 an "early prototype" and a "primitive version" of the SCL-90-R,2 the final version of the scale. In fact, the scales are similar; only several minor modifications were made in the SCL-90. Eighty-seven of the 90 items are identical; SCL-90 item 3 was reworded and items 80 and 86 were changed. The SCL-90 asks the patient "how much were you bothered by..." while the SCL-90-R asks "how much were you distressed by.... " Derogatis incorrectly stated that the instruments are scaled differently; they use the same five-point response scale with identical descriptors: "not at all, a little bit, moderately, quite a bit, extremely."Derogatis asserted that the nine Analogue descriptors correspond to the item content of the respective nine SCL-90-R factors, not the SCL-90 factors. Yet, the items are identical for seven of the nine factors: somatization, interpersonal sensitivity, depression, hostility, phobic anxiety, paranoid ideation,