Introduction In statistics, Mallows's Cp, named for Colin Lingwood Mallows, an English statistician, is used to assess the fit of a regression model that has been estimated using ordinary least squares. Models with a Mallows' Cp value near P+1 (i.e. the number of explanatory variables + 1) have a low bias. If every potential model has a high value for Mallows' Cp, this indicates that some important predictor variables are likely missing from each model.