  The goal of positive-unlabeled (PU) learning is to train a binary classifier
on the basis of training data containing positive and unlabeled instances,
where unlabeled observations can belong either to the positive class or to the
negative class. Modeling PU data requires certain assumptions on the labeling
mechanism that describes which positive observations are assigned a label. The
simplest assumption, considered in early works, is SCAR (Selected Completely at
Random Assumption), according to which the propensity score function, defined
as the probability of assigning a label to a positive observation, is constant.
On the other hand, a much more realistic assumption is SAR (Selected at
Random), which states that the propensity function solely depends on the
observed feature vector. SCAR-based algorithms are much simpler and
computationally much faster compared to SAR-based algorithms, which usually
require challenging estimation of the propensity score. In this work, we
propose a relatively simple and computationally fast test that can be used to
determine whether the observed data meet the SCAR assumption. Our test is based
on generating artificial labels conforming to the SCAR case, which in turn
allows to mimic the distribution of the test statistic under the null
hypothesis of SCAR. We justify our method theoretically. In experiments, we
demonstrate that the test successfully detects various deviations from SCAR
scenario and at the same time it is possible to effectively control the type I
error. The proposed test can be recommended as a pre-processing step to decide
which final PU algorithm to choose in cases when nature of labeling mechanism
is not known.
