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Table 2 Magnitude of interactions for the 16 studies reporting mean costs and mean health benefits for each cell within the factorial design

From: Which interactions matter in economic evaluations? A systematic review and simulation study

 Interaction: effect ratioProportion of interactions in different categories
CostHealth benefitNet monetary benefita
Qualitative<−116.7% (4/24)33.3% (8/24)29.2% (7/24)
Sub-additiveBetween −1 and 041.7% (10/24)25.0% (6/24)25.0% (6/24)
Mixed non-qualitativeb4.2% (1/24)12.5% (3/24)12.5% (3/24)
Additive00.0% (0/24)8.3% (2/24)0.0% (0/24)
Super-additive> 037.5% (9/24)20.8% (5/24)33.3% (8/24)
Interaction:effect ratio > 1 or < −129% (7/24)38% (9/24)38% (9/24)
Range of interaction:effect ratios-2, 232−23, 2−44, 13
  1. a A ceiling ratio of £20,000 (or $20,000 or €20,000) was used for quality-adjusted life-years (QALYs) [14] and life-years gained, while a ceiling ratio of £5000 (or $5000 or €5000) per unit of benefit was arbitrarily used for all other health benefits
  2. b Mixed interaction: one factor increases the outcome of interest, while the other decreases it. The interaction therefore cannot be classified as either sub-additive or super-additive