What is a superiority trial?

Researchers investigated the efficacy of agnus castus fruit (Vitex agnus castus L extract Ze 440) in relieving symptoms of the premenstrual syndrome. A randomised, double blind, placebo controlled, parallel group, superiority trial was performed. The intervention was agnus castus (dry extract tablets), with one tablet daily for three consecutive cycles.1

Participants were 170 women with diagnosed premenstrual syndrome recruited from general medicine community clinics, with a mean age of 36 years, mean cycle length 28 days, and a mean duration of menses 4.5 days. In total 86 women were randomised to intervention and 84 to placebo. The primary outcome variable was the combined scores of self assessment of six symptoms: irritability, mood alteration, anger, headache, other menstrual symptoms (including bloating), and breast fullness. Participants used a visual analogue scale to rate each symptom. The scale, validated for the assessment of the premenstrual syndrome, was of length 10 cm, ranging from 0 (no symptoms) to 10 (unbearable). The primary outcome was recorded at baseline and at the end of the third menstrual cycle, and the change in total score was recorded for each woman.

The mean reduction in the primary outcome was significantly greater in the intervention group (mean reduction 128.5 mm) than in the placebo group (78.1 mm) (mean difference 50.5 mm (95% confidence interval 23.5 mm to 77.5 mm; P< 0.001)). The researchers concluded that dry extract of agnus castus …