Linear Classifiers Under Infinite Imbalance
This work addresses classification under extreme class imbalance, a common problem in domains like credit risk, but it is incremental as it extends prior results on logistic regression to a broader class of linear classifiers.
The paper analyzes linear classifiers in the infinite-imbalance limit, where one class's sample size grows indefinitely, showing that the intercept diverges but other coefficients converge to finite limits depending on the weight function's tail growth, with applications in credit risk focusing on high-sensitivity and high-specificity performance.
We study the behavior of linear discriminant functions for binary classification in the infinite-imbalance limit, where the sample size of one class grows without bound while the sample size of the other remains fixed. The coefficients of the classifier minimize an empirical loss specified through a weight function. We show that for a broad class of weight functions, the intercept diverges but the rest of the coefficient vector has a finite almost sure limit under infinite imbalance, extending prior work on logistic regression. The limit depends on the left-tail growth rate of the weight function, for which we distinguish two cases: subexponential and exponential. The limiting coefficient vectors reflect robustness or conservatism properties in the sense that they optimize against certain worst-case alternatives. In the subexponential case, the limit is equivalent to an implicit choice of upsampling distribution for the minority class. We apply these ideas in a credit risk setting, with particular emphasis on performance in the high-sensitivity and high-specificity regions.