For which of the following models should external validation be used? Consider both the population used to train the model, and the population that the model will be used on. (Select all that apply.)
In the first and third models, we are using a special sub-population to build the model. While we can use the model for that sub-population, we should use external validation to test the model on other populations. The second model uses data from a special sub-population, but the model is only intended for that sub-population, so external validation is not necessary.