3. What is cross-validation method for estimating validity in prediction?

a. using coefficients from one sample to predict outcome in a second sample and, subsequently, using the correlation between the predicted and observed to estimate validity

b. using the correlation between 2 predictors to calculate the unique contribution of the predictor and to estimate the outcome variable form residual value of the predictor

c. using the average correlation of outcome variable and predictors in the full sample to estimate the shrunken R in a 20% sample

d. none of the above