Scribble a list of test ideas down the y-axis and list aspects of some test coverage model—product elements, quality criteria, platforms, test techniques—across the x-axis. As an experiment, create multiple sheets using the same tests, but with a different coverage model on each sheet, and observe how a single test can provide coverage in a number of different dimensions. For a given coverage model, denser coverage of the matrix suggests (but does not prove) deeper coverage of the particular set of ideas on that table.