How to assess internal validity or causality?

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Internal validity is the extent to which the observed results represent the truth in the population and are not due to methodological errors or other alternative explanations. To justify the causal relationship between the treatment and the outcome, we have to check whether the association between the treatment and the outcome is robust across several estimation methods, whether the treatment happened before the outcome, and whether there exist confounders causing the co-variation of the treatment and the outcome.