The Components of Internal Validity Are A $R ^ { 2 }$

Short Answer

The components of internal validity are a. a large sample, and BLUE property of the estimator.
b. a regression $R ^ { 2 }$ above $0.75$ and serially uncorrelated errors.
c. unbiasedness and consistency of the estimator, and desired significance level of hypothesis testing.
d. nonstochastic explanatory variables, and prediction intervals close to the sample mean.