An audience measurement system generates signatures of unknown pieces of content viewed by panel members, and generates reference signatures of known pieces of content. The signatures of the unknown pieces of content are stored and transmitted to a central processing site, where they are compared with the reference signatures for their identification. A scanning engine finds matches between the signatures of the unknown and the known contents, and stores matches so as to build “tracking segments”, i.e., strings of matches that indicate a coincidence between the unknown content and one or more known pieces of content for a certain period of time, and associates the unknown content to the known piece of content with the longest tracking segment.