title: 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.
abstract: 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.
authors.affiliation: Hugh Sinclair Unit of Human Nutrition, The School of Food Biosciences, The University of Reading, Reading RG6 6AP, UK.
authors.forename: Stephen M
authors.initials: SM
authors.lastname: Hicks
authors.forename: Ann F
authors.initials: AF
authors.lastname: Walker
authors.forename: James
authors.initials: J
authors.lastname: Gallagher
authors.forename: Richard W
authors.initials: RW
authors.lastname: Middleton
authors.forename: John
authors.initials: J
authors.lastname: Wright
mesh_terms: 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
chemical_list: D014151:Anti-Anxiety Agents; D000928:Antidepressive Agents; D010936:Plant Extracts