why is it better to perform cross validation or test why is cross validation performed on a sample is cross validation good or bad why is cross validation useful in the a/k test when are a model set in cv or validation why is cross validation good why is cross validation used? what is the difference between an internal and external validation what does internal validation mean why do they do internal validation why is it better to do a holdout test set or an interval why is internal validation better than repeating testing why is cross validation performed why is an external model selection useful does the internal validity test reduce the variance of the estimater? what is the difference between internal and external cross validation why is the k-fold set a bad practice why is it better to do external and internal validation and model selection why is it useful to re-validate an estimator why is external vs internal model selection important