Nearest Neighbour Classifiers for Streaming Data with Delayed Labelling

We study streaming data where the true labels come with a delay. The question is whether the online nearest neighbour classifier (IB2 and IB3 here) should employ the unlabelled data. Three strategies are examined: do-nothing, replace and forget. Experiments with 28 data sets show that IB2 benefits from unlabelled data, while IB3 does not.