Table 3 Predictors of placebo effect size for pain
Variablesβ (95% CI)p Value
EStreatment0.3802 (0.2545 to 0.5059)<0.001
Percentage baseline pain (0–100)0.0055 (0.0012 to 0.0099)0.014
Sample size0.0007 (0.0003 to 0.0012)0.004
Invasive treatment (yes = 1, no = 0)*0.1436 (0.0247 to 0.2625)0.020
  • Other variable considered but excluded from the model due to insignificance included age (years), gender (F/M), BMI (kg/m2), intention-to-treat analysis (yes/no), blindness (yes/no), community trial (yes/no), trials funded by industry (yes/no), trial conducted in the US (yes/no), target joint (hand = 4, knee = 3, hip & knee = 2 and hip = 1) and topical therapy (yes/no). Stepwise approach was used for the calculation. β is the partial regression coefficient.

  • *Invasive treatment included acupuncture, intra-articular, intramuscular, intravenous injections and surgical therapies.

  • BMI, body mass index; ES, effect size.