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Letters to the Editor
May 2002

Limitations of the Bonn Scale for the Assessment of Basic Symptoms as a Screening Measure

Arch Gen Psychiatry. 2002;59(5):471. doi:

In reply

In naturalistic sampling, measures of diagnostic or prognostic accuracy, such as sensitivity, specificity, and positive and negative predictive value, are somewhat biased depending on the sample size, "N."1 This bias is only negligible if N is large enough relative to the prevalence of the disorder, "P," and the level of test, "Q," (ie, the number of observations in each marginal position of the 2 × 2 table must be reasonably large). As a rule of thumb, Kraemer1 recommended that "no test evaluation be undertaken unless one can expect to see at least 10 patients in each of the marginal positions of a table."1(p39) Thus, based solely on the lifetime prevalence for schizophrenia of 1%, more than 1000 persons had to be included; based on the incidence of 0.0001% or 1/10 000 and an average follow-up period of 10 years, even more than 10 000 persons had to be included for a naturalistic sampling of the general population.

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