A few useful perspectives and experiments related to detecting rare events. I reproduce the papers’ abstracts (or at least part of the abstracts) below.

Data Set Balancing, David L. Olson

*This paper conducts experiments with three skewed data sets, seeking to demonstrate problems when skewed data is used, and identifying counter problems when data is balanced. The basic data mining algorithms of decision tree, regression-based, and neural network models are considered, using both categorical and continuous data. Two of the data sets have binary outcomes, while the third has a set of four possible outcomes. Key findings are that when the data is highly unbalanced, algorithms tend to degenerate by assigning all cases to the most common outcome. When data is balanced, accuracy rates tend to decline. If data is balanced, that reduces the training set size, and can lead to the degeneracy of model failure through omission of cases encountered in the test set. Decision tree algorithms were found to be the most robust with respect to the degree of balancing applied.*

Sampling Bias and Class Imbalance in Maximum-likelihood Logistic Regression, Oomen et al.

*We hypothesize that the predictive capability of the model is related to the sampling bias associated with the data so that the MLLR [LF: maximum likelihood logistic regression] model has perfect predictability when the data have no sampling bias. We test our hypotheses using two simulated datasets with class distributions that are 50:50 and 80:20, respectively. We construct a suite of controlled experiments by extracting multiple samples with varying class imbalance and sampling bias from the two simulated datasets and fitting MLLR models to each of these samples. The experiments suggest that it is important to develop a sample that has the same class distribution as the original population rather than ensuring that the classes are balanced. Furthermore, when sampling bias is reduced either by using over-sampling or under-sampling, both sampling techniques can improve the predictive capability of an MLLR model.*

Logistic Regression in Rare Events Dat, by Gary Kind and Lanche Zeng

*We study rare events data, binary dependent variables with dozens to thousands of times fewer ones (events, such as wars, vetoes, cases of political activism, or epidemiological infections) than zeros (“nonevents”). In many literatures, these variables have proven difficult to explain and predict, a problem that seems to have at least two sources. First, popular statistical procedures, such as logistic regression, can sharply underestimate the probability of rare events. We recommend corrections that outperform existing methods and change the estimates of absolute and relative risks by as much as some estimated effects reported in the literature. Second, commonly used data collection strategies are grossly inefficient for rare events data. The fear of collecting data with too few events has led to data collections with huge numbers of observations but relatively few, and poorly measured, explanatory variables, such as in international conflict data with more than a quarter-million dyads, only a few of which are at war. As it turns out, more efficient sampling designs exist for making valid inferences, such as sampling all available events (e.g., wars) and a tiny fraction on nonevents (peace). This enables scholars to save as much as 99% of their (nonfixed) data collection costs or to collect much more meaningful explanatory variables. We provide methods that link these two results, enabling both types of corrections to work simultaneously, and software that implements the methods developed.*