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J Altern Complement Med. 2004 Dec;10(6):925-32.

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.

Author information

1
Hugh Sinclair Unit of Human Nutrition, The School of Food Biosciences, The University of Reading, Reading RG6 6AP, UK.

Abstract

OBJECTIVES:

This study aimed to investigate the efficacy of St. John's wort extract (SJW) as a treatment for premenstrual symptoms.

DESIGN:

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.

SETTINGS/LOCATION:

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

SUBJECTS:

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.

INTERVENTIONS:

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

OUTCOME MEASURE:

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.

CONCLUSION:

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.

PMID:
15673985
DOI:
10.1089/acm.2004.10.925
[Indexed for MEDLINE]

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