All models are wrong; some are useful.Abraham Lincoln

Model selection is the problem of choosing the *best* model among a set of competing models. There are several dimensions that can help us define what *best* means, such as:

- Sensitivity: we want all the important parameters to capture all the complexity of the data.
- Specificity/parsimony: we favor simpler models, with fewer parameters, to avoid overfitting.
- Future predictive ability: we want the model to generalize well.
- Selection consistency: the model size converges in probability to the true model size.

Usually we require a trade-off between different terms. That is because different approaches take emphasize different dimensions: cross-validation, for instance, focuses on future predictive ability. For example, we typically balance goodness of fit (how well the model describes the data) and parsimony (to avoid over-fitting). It can be interpreted as the trade-off between the bias introduced by a model too small and the variance that comes by a model too large. Itâ€™s worth mentioning that the true model is unbiased and only has the necessary variance.

- Hastie, T., Tibshirani, R., & Friedman, J. (2009). The Elements of Statistical Learning. New York, NY: Springer New York. https://doi.org/10.1007/978-0-387-84858-7