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No long-term benefit for cognitive therapy in acute psychosis: a type II error

Lambeth Mental Health Services, South London and Maudsley NHS Trust, 108 Landor Road, London SW9 9NT

EDITED BY MATTHEW HOTOPF

Drury et al (2000) reported no significant difference in relapse rates, positive symptoms or insight between a cognitive therapy group and a recreational activities and support group of patients who had an acute episode of a non-affective psychosis. This 5-year outcome study assessed 34 out of an original cohort of 40 patients.

Working on the basis of small trials having a large type II error, the group size for each group can be estimated. If the anticipated mean response in one group is µ₁ and the standard deviation is , to show a significant result the mean relapse of one group can be estimated at 2 (µ₁) and the standard deviation can be estimated at 1.5 (). The estimated difference between the groups () can be set at 0.5 (µ₂-µ₁). A formula to calculate the number (n) in each group (Pocock, 1983: 127-128) can be used as follows:

The (type I error) is by convention set at 0.05, and the ß (type II error) can be set at 0.2. The power of finding a true result (1 - ß) will therefore be 0.8 or 80% and, by using a statistical table, f(, ß) is 7.9. Therefore, n can be calculated as

patients in each group.

It would therefore take a very large sample to prove the null hypothesis in the above hypothetical estimate. In the study by Drury et al (2000), it would be misleading to extrapolate that there was no long-term benefit of using cognitive therapy in schizophrenia in terms of relapse. Larger studies are needed in this rapidly evolving area.

REFERENCES

Drury, V., Birchwood, M. & Cochrane, R. (2000) Cognitive therapy and recovery from acute psychosis: a controlled trial. 3. Five-year follow-up. British Journal of Psychiatry, 177, 8-14.[Abstract/Free Full Text]

Pocock, S. (1983) Clinical Trials: A Practical Approach. Chichester: John Wiley & Sons.

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This Article Submit an eLetter Alert me when this article is cited Alert me when eLetters are posted Alert me if a correction is posted Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via CrossRef Citing Articles via Google Scholar Google Scholar Articles by Marlowe, K. Search for Related Content PubMed PubMed Citation Articles by Marlowe, K.