Table 2.

 Quality of evidence is weaker if comparisons in trials are indirect

Question of interest	Source of indirectness
Relative effectiveness of alendronate and risedronate in osteoporosis	Indirect comparison: randomised trials have compared alendronate with placebo and risedronate with placebo, but trials comparing alendronate with risedronate are unavailable
Oseltamivir for prophylaxis of avian flu caused by influenza A (H5N1) virus	Differences in population: randomised trials of oseltamivir are available for seasonal influenza, but not for avian flu
Sigmoidoscopic screening for prevention of mortality from colon cancer	Differences in intervention: randomised trials of faecal occult blood screening provide indirect evidence, bearing on potential effectiveness of sigmoidoscopy
Choice of drug for schizophrenia	Differences in comparator: series of trials comparing newer generation neuroleptic agents with fixed doses of haloperidol 20 mg provide indirect evidence of how newer agents would compare with lower, flexible doses of haloperidol that clinicians typically use
Rosiglitazone for prevention of diabetic complications in patients at high risk of diabetes	Differences in outcome: randomised trial shows delay in development of biochemical diabetes with rosiglitazone but was underpowered to tackle diabetic complications