Internal validity is a measure of how well a study is conducted and how accurately its results reflect the studied group. It is the extent to which a piece of evidence supports a claim about cause and effect within the context of a particular study. Internal validity is determined by how well a study can rule out alternative explanations for its findings, usually sources of systematic error or bias. In other words, internal validity is the degree of confidence that the causal relationship being tested is not influenced by other factors or variables. A valid causal inference may be made when three criteria are satisfied: the "cause" precedes the "effect" in time (temporal precedence), the "cause" and the "effect" tend to occur together (covariation), and there is no plausible alternative explanation for the relationship.

Internal validity is one of the most important properties of scientific studies and is an important concept in reasoning about evidence more generally. It makes the conclusions of a causal relationship credible and trustworthy. However, there is an inherent trade-off between internal and external validity. The more extraneous factors are controlled in a study, the less the findings can be generalized to a broader context.

Threats to internal validity include selection bias, performance bias, detection bias, and attrition bias. To increase internal validity, investigators should ensure careful study planning and adequate quality control and implementation strategies, including adequate recruitment strategies, data collection, data analysis, and sample size.