The significance of "nonsignificance" in randomized controlled studies: a discussion inspired by a double-blinded study on St. John's wort (Hypericum perforatum L.) for premenstrual symptoms.
OBJECTIVE
This study aimed to investigate the efficacy of St. John's wort extract (SJW) as a treatment for premenstrual symptoms.


METHODS
The study was a randomized, double-blinded, placebo-controlled trial, with two parallel treatment groups. After a no-treatment baseline cycle, volunteers were randomized to either SJW or placebo for a further two menstrual cycles.


METHODS
A postal trial conducted from The University of Reading, Berkshire, England.


METHODS
One hundred and sixty-nine (169) normally menstruating women who experienced recurrent premenstrual symptoms were recruited onto the study. One hundred and twenty-five (125) completed the protocol and were included in the analysis.


METHODS
Six hundred milligrams (600) mg of SJW (standardized to contain 1800 microg of hypericin) or placebo (containing lactose and cellulose).


METHODS
A menstrual diary was used to assess changes in premenstrual symptoms. The anxiety-related subgroup of symptoms of this instrument was used as the primary outcome measure.


RESULTS
After averaging the effects of treatment over both treatment cycles it was found that there was a trend for SJW to be superior to placebo. However, this finding was not statistically significant.


CONCLUSIONS
The possibility that this nonsignificant finding resulted from insufficient statistical power in the study, rather than a lack of efficacy of SJW, is discussed. Following this discussion the recommendation is made that, in future, similar studies should be powered to detect a minimum clinically relevant difference between treatments.
Hicks
Stephen M
SM
Hugh Sinclair Unit of Human Nutrition, The School of Food Biosciences, The University of Reading, Reading RG6 6AP, UK.
Walker
Ann F
AF
Gallagher
James
J
Middleton
Richard W
RW
Wright
John
J
D000328:Adult; D014151:Anti-Anxiety Agents / Q000627:therapeutic use; D000928:Antidepressive Agents / Q000627:therapeutic use*; D001007:Anxiety / Q000188:drug therapy; D003863:Depression / Q000188:drug therapy; D004305:Dose-Response Relationship, Drug; D004311:Double-Blind Method; D004334:Drug Administration Schedule; D004739:England; D005260:Female; D006801:Humans; D020902:Hypericum; D008517:Phytotherapy; D010936:Plant Extracts / Q000627:therapeutic use; D011293:Premenstrual Syndrome / Q000188:drug therapy* / Q000517:prevention & control; D012044:Regression Analysis; D016896:Treatment Outcome
D014151:Anti-Anxiety Agents; D000928:Antidepressive Agents; D010936:Plant Extracts