Abstract: A method of determining whether particular data is included in a data store. The particular data comprises a plurality of first data values and the data store comprises a plurality of second data values. The method comprises obtaining identification data associated with the particular data. The identification comprises a subset of said first data values. A portion of the data store is retrieved and it is determined whether at least a predetermined number of data values of the identification data have a predetermined relationship with corresponding second data values of the retrieved portion of the data store.