1. A method of processing data of a data set, wherein the data set comprises a plurality of data groups, the method comprising: for the data of each one of the data groups, mapping the data of that data group to corresponding intermediate data for that data group and identifiable to that data group; and reducing the intermediate data for the data groups to at least one output data group, including processing the intermediate data for each data group in a manner that is defined to correspond to that data group.