The House of Commons also has several departmental select committees. The membership of these bodies, like that of the standing committees, reflects the strength of the parties. The chairman of each committee is voted on in a secret ballot of the whole house during the first session of a parliamentary term, or when a vacancy occurs. The number of select committee chairmanships allocated to each party reflects the strength of the parties, and the parties allocate the positions through agreement. The primary function of a departmental select committee is to scrutinise and investigate the activities of a particular government department. To fulfil these aims, it is permitted to hold hearings and collect evidence. Bills may be referred to Departmental Select Committees, but such a procedure is seldom used.