1. A medical image data processing apparatus comprising processing circuitry configured to: receive a plurality of sets of medical imaging data; and train a classifier for use in classification, wherein the training of the classifier comprises, for each of the plurality of sets of medical imaging data: selecting a first part and a second part of the respective set of medical imaging data, wherein the first part and the second part are representative of different regions of the same subject; and training the classifier for use in classification based on the first part of the set of medical imaging data and the second part of the set of medical imaging data, and the training of the classifier for use in classification based on the first part of the set of medical imaging data and the second part of the set of medical imaging data comprises training the classifier for use in determining presence or absence of at least one abnormality in the first part of the set of medical imaging data based at least partially on the second part of the set of medical imaging data and ground truth data associated with the first part of the set of medical imaging data.