Abstract: The invention relates to a data medium (1) comprising an electronic chip provided with an identity zone storing identity data relating to the entity (6a, 6b) and a group zone storing group data relating to the membership of the entity (6a, 6b) to at least one group (5a, 5b). The invention also relates to a device (10) for controlling entities, comprising: data media (1); an identity control unit (11) enabled and able to recognize the identity data; an updating unit (12) enabled and able to update the group data as a function of the movement of the entity (6a, 6b) from one group (5a, 5b) to another, in order to generate a log; a group control unit (13) authorised and able to control the group data of each entity (6a, 6b); and alert means that can be triggered as a function of said controlled group data.