Table 3 Factors explaining placebo and intervention effects
Univariate | No. of RCTs | ß (P-value) on placebo effect* | ß (P-value) on intervention effect* |
|---|---|---|---|
Adverse events: | |||
    % Headache in placebo group | 48 | -0.11 (0.469) | 0.02 (0.918) |
    % Headache in intervention group | 48 | -0.28 (0.052) | -0.11 (0.453) |
    Risk ratio for headache | 48 | -0.08 (0.569) | -0.11 (0.472) |
    % Flushing in placebo group | 37 | 0.24 (0.135) | 0.02 (0.917) |
    % Flushing in intervention group | 37 | 0.10 (0.559) | 0.30 (0.071) |
    Risk ratio for flushing | 37 | -0.17 (0.278) | 0.21 (0.207) |
Study related: | |||
    Many follow-ups (≥4 vs 1 or 2) | 21 | 0.05 (0.823) | -0.04 (0.867) |
    Sample size | 61 | -0.012 (0.928) | 0.21 (0.098) |
    Long duration of double blind (>12 weeks versus ≤4 weeks) | 15 | 0.56 (0.005) | 0.52 (0.025) |
    % Randomized not analyzed | 43 | 0.43 (0.002) | 0.25 (0.102) |
    ITT analysis (yes vs no) | 51 | -0.07 (0.638) | -0.23 (0.094) |
    Publication year | 61 | 0.02 (0.881) | -0.42 (<0.001) |
    Parallel study design (yes vs no) | 61 | -0.10 (0.451) | -0.07 (0.589) |
    Study run-in placebo | 2 | NA | NA |
    Commercial funding (yes vs no) | 36 | -0.09 (0.594) | -0.04 (834) |
    Continent (North America = ref)** | 47 |  |  |
    Across continents | 19 | 0.36 (0.038) | 0.16 (0.392) |
    Asia | 6 | 0.42 (0.005) | -0.07 (0.667) |
    Europe | 9 | 0.25 (0.137) | 0.21 (0.249) |
    Single center study (yes vs no) | 56 | 0.14 (0.308) | -0.07 (0.623) |
    Type of PDE-5 inhibitor (tadalafil = ref) | 59 |  |  |
    Sildenafil | 30 | 0.24 (0.105) | 0.11 (0.474) |
    Vardenafil | 10 | 0.17 (0.259) | 0.19 (0.199) |
    % Prior experience with intervention | 14 | -0.55 (0.013) | -0.05 (0.874) |
Patient related: | Â | Â | Â |
    Prior experience with intervention (yes vs no) | 9 | -0.86 (<0.001) | -0.10 (0.772) |
    % Psychogenic etiology | 40 | 0.11 (0.485) | -0.23 (0.150) |
    Prostate cancer or spinal cord injury (yes vs no) | 19 | 0.04 (0.854) | -0.59 (0.003) |
    Baseline disease severity*** | 58 | -0.31 (<0.001) | -0.44 (<0.001) |
    Disease duration | 29 | 0.06 (0.761) | -0.07 (0.714) |
Multivariate | No. of RCTs | ß ( P -value) |  |
Model on placebo effect (76% explained) | 10**** | Â | Â |
Naïve to intervention (yes vs no) |  | 0.52 (0.012) |  |
% Randomized not analyzed in placebo groups | Â | 0.44 (0.033) | Â |
Model on intervention effect (40% explained) | 107 | Â | Â |
Placebo effect | Â | 0.32 (<0.001) | Â |
Baseline disease severity*** | Â | -0.33 (<0.001) | Â |
Year of publication | Â | -0.26 (0.002) | Â |
Sample size | Â | 0.23 (0.003) | Â |