Data items are selected for inclusion in a data set so as to discourage a transmission of the entire set. Each data item includes section(s) which constitute(s) the complete data set. Each section contains a watermark that includes an identifier of the section, and an identifier of the data set. The identifier of the section may be the address of the section, and the identifier of the data set may be the serial number and an indicator of the total size of the data set. The presence of the data set is confirmed by checking the watermarks of randomly selected sections to verify that the original section that formed the data set is present. If a section is discovered to be missing or altered, subsequent processing of data items of the data set is prevented. The identifiers may be stored as a combination of robust and fragile watermarks.