Abstract: We describe an RFID communications network comprising: a plurality of passive RFID tags; at least one RF power supply device; and at least one RFID reader; and wherein a said RFID tag is configured to use a first frequency to receive power for powering said tag from said RF power supply device and to use a second, different frequency to communicate with said RFID reader; and wherein said plurality of RFID tags are configured to use the same said first frequency to receive said power, and all use different second frequencies to communicate with said RFID reader. The tags may be used to generate probability data, the probability data for a said identified tag being dependent upon a probability of the tag belonging to a collection the contents of which are to be verified; contents of the collection may then be verified against a schedule using processed verification data.