Abstract: A method and apparatus is disclosed for producing comparative data in a visually assimilable form. The method comprises providing a substrate which is divided into visually distinct zones each representative of a different area of comparison, providing first means over said substrate aligned with at least one said zone and visually indicative of first data relative to said at least one zone, and providing second means over said first means aligned with at least one said zone and visually indicative of second data relevant to said at least one zone, said second means being visually different to said first means. Said first and second means where they coincide in a said zone combine to provide third means visually different to said first and second means, whereby to provide an immediate visual comparison between the data represented by said first and second means for each of said zones.