1. A semiconductor wafer critical dimension measurement method, comprising receiving an image of a site of the semiconductor wafer comprising a plurality of features, processing the image to measure at least one critical dimension of at least some of the features, analysing the critical dimension of each feature and determining the feature to be a non-defective feature or a defective feature, and using the critical dimensions of at least some of any non-defective features as a measure of the critical dimensions of features of the semiconductor wafer, wherein analysing the critical dimension of a feature and determining the feature to be a non-defective feature or a defective feature, comprises comparing the critical dimension of the feature with the critical dimensions of a plurality of other features and determining the feature to be a non-defective feature if the critical dimension of the feature is substantially equal to the critical dimensions of a majority of the other features, and determining the feature to be a defective feature if the critical dimension of the feature is not substantially equal to the critical dimensions of a majority of the other features.