period of time when the multimedia presenting device 3 is on and a panel member has registered his or her presence an example of such a system is described in the applicant 39 s co pending international patent application published under no 2008 072094 which is herein incorporated by reference fig2 shows a schematic diagram of a metered viewing session in the example shown in fig2 a viewing session 10 starting at a time t 1 and ending at a time t 2 is represented each viewing session in turn is divided into u201c viewing segments u201d 21 22 23 24 25 in the figure i e a period of time wherein the same broadcast source is tuned into at the multimedia presenting device 3 the minimum length of time that is considered by the audience measurement system 1 as a viewing session is called u201c persistence threshold u201d and its value is defined during the system set up process a value of fifteen seconds is used in most countries and will be assumed in the description that follows in the case of content matching systems each metering device 2 generates signatures 30 of the content present during the viewing segments and the signatures 30 are then sent to a central processing site.