This article presents an unified approach of several procedures in time series. First, we show that quadratic discrimination provides a unifying approach for deriving model selection criteria in the ARMA framework. Second, we establish a connection between model selection criteria and a goodness of fit test. Finally, we show that the outlier detection problem can be seen as a particular case of model selection. Therefore, the problems of model selection, discrimination, goodness of fit tests and outliers can be treated under the same principles.