data reduction

Sometimes we have so much data it cannot be processed in its entirity, in which case some form of data reduuction is necessary. This may be about educing the number of data items and/or reducing the size of eacj item. The nunber of data items may be reduced by filtering so that only a subset of the dataitems are used, or in some way grouping items, for example working with area averages for geograohic data. Similarly we might choose to only use a subset of features in each data item, combine features, or process complex features (such as images).