Instead of choosing the penalty parameter to balance model fit with model complexity, cross-validation requires it (i.e. the penalty parameter) to yield a model with good prediction performance. Commonly, this performance is evaluated on novel data. Novel data need not be easy to come by and one has to make do with the data at hand. The setting of `original' and novel data is then mimicked by sample splitting: the data set is divided into two (groups of samples). One of these two data sets, called the training set, plays the role of `original' data on which the model is built. The second of these data sets, called the test set, plays the role of the `novel' data and is used to evaluate the prediction performance (often operationalized as the log-likelihood or the prediction error or its square or the R2 score) of the model built on the training data set. This procedure (model building and prediction evaluation on training and test set, respectively) is done for a collection of possible penalty parameter choices. The penalty parameter that yields the model with the best prediction performance is to be preferred. The thus obtained performance evaluation depends on the actual split of the data set. To remove this dependence the data set is split many times into a training and test set. For each split the model parameters are estimated for all choices of \( \lambda \) using the training data and estimated parameters are evaluated on the corresponding test set. The penalty parameter that on average over the test sets performs best (in some sense) is then selected.