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Light therapy for seasonal affective disorder: a type II error

Coventry Healthcare NHS Trust, The Caludon Centre, Clifford Bridge Road, Walsgrave, Coventry CV2 2TE, UK

A. N. Singh

Division of Psychopharmacology, Queen's University and Kingston Psychiatric Hospital, Kingston, Ontario, Canada

N. Soni

Coventry Healthcare NHS Trust, Coventry, UK

EDITED BY MATTHEW HOTOPF

Wileman et al (2001) evaluated bright white v. dim red light therapy for seasonal affective disorder in primary care and reported no significant difference in the proportions of responders in either group. Working on the basis of small trials having a large type II error, the group size for each group can be estimated. For most statistical tests, tables are available that show the power of the test to detect specified differences for a given and sample size, as well as tables that show the required sample size to achieve selected power for specified difference and given (Machin & Campbell, 1987; Cohen, 1988). In designing a clinical trial, we should select the power 1 — ß to be at least 0.80, so that there is a chance of one in five or less of missing an important difference between treatments.

The differences in response rates between the two groups even using the broad remission criterion was 16.9%. To detect this difference at a significance level =0.05, power 80%, 160 patients are required in each group (Freeman & Tyrer, 1992). However, only 57 patients in total were enrolled by Wileman et al. This reduced the power of the tests to about 20% and strongly suggests a type II error. It would therefore be misleading to conclude that bright white light is not associated with greater improvement.

REFERENCES

Cohen, J. (1988) Statistical Power Analysis for the Behavioral Sciences (2nd edn). Hillsdale, NJ: L. Erlbaum.

Freeman, C. & Tyrer, P. (eds) (1992) Research Methods in Psychiatry (2nd edn), p. 52. London: Gaskell.

Machin, D. & Campbell, M. J. (1987) Statistical Tables for the Design of Clinical Trials. Oxford: Blackwell Scientific.

Wileman, S. M., Eagles, J. M., Andrew, J. E., et al (2001) Light therapy for seasonal affective disorder in primary care. Randomised controlled trial. British Journal of Psychiatry, 178, 311-316.[Abstract/Free Full Text]

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 CrossRef Citing Articles via Google Scholar Google Scholar Articles by Jainer, A. K. Articles by Soni, N. Search for Related Content PubMed PubMed Citation Articles by Jainer, A. K. Articles by Soni, N.