The internal validity of a model, such as multivariate regression, looks at whether the independent variables used are statistically significant; that is, are the internal constructs of the model valid? We typically use the p-value from regression to measure this internal validity. The null hypothesis tested is that each of the independent variables has zero effect on the dependent variable. Hence, low p-values, at or below the alpha significance level, imply that it is statistically significant and impacts the dependent variable. Hence, a model with only significant independent variables is deemed internally valid.