Abstract
The detection of influential observations is important because of their unduly large influence on the regression analysis results. Numerous diagnostics on identifying these observations are developed in the regression analysis. Pena's statistic is one of the proposed diagnostics. In this study, Pena's approach is formulated to ridge regression (RR) and modified ridge regression (MRR). The real data and artificial data are used to see performance of proposed diagnostic for the RR and MRR on detecting influential observations.