When trying to assess the effects of interventions, we should look for valid evidence which is relevant to our needs. This may be provided in an evidence-based guideline. The evidence should describe outcomes that are person-centred. They should tell you about survival or quality of life rather than surrogate measures. The evidence should describe both harms and benefits and tell you how probable they are.
Studies vary in their reliability as a source of evidence.
The following criteria will help you determine whether a relationship is causal. The evidence comes from the best study type. The exposure to the supposed cause occurred before the outcome. The relationship between supposed cause and outcome is strong. The relationship cannot be explained by any other factors.