The statistical power of 61 negative clinical trials of therapeutic regimens in patients with cystic fibrosis published from 1977 through 1988 was reviewed and the ability of the investigations to detect small, medium, and large standardized differences was calculated. Small, medium, and large standardized differences were defined as ratios of 0.2, 0.5, and 0.8, respectively, of the observed difference compared with the standard deviation. The average numbers (±SD) of patients in the treatment and control groups were 14.3 ± 6.9 and 14.5 ± 7.9, respectively. None of the studies had 80% power to detect a small or medium standardized difference and only 4 of the reports had 80% statistical power to detect a large standardized difference. The variability of cystic fibrosis causes a decrease in the standardized difference, making it more difficult to demonstrate statistical significance. Statistical power of negative clinical trials reported in the literature deserves more attention from investigators as well as physicians who treat patients with cystic fibrosis.

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