Internal validity

The internal validity or credibility of the study is the capacity of the results to support a causality. In other words, the observed difference in outcome between groups is attributable only to the effect of the intervention under investigation. The internal validity is assessed by adequacy of allocation sequence, concealment of allocation sequence, blinding, the balance of prognostic factors, inten-to-treat analysis, and completeness of follow-up. The reviewers need to have the ability to recognize the relationship between an explanatory factor and an outcome and to identify other variables (confounding factors) or systematic error (bias) that can distort the results. Confounding factors and study bias are different problems in the study that lead to an inaccurate estimate (underestimate or overestimate) of the true association between exposure and an outcome ¹⁾.