1. A method of determining a characteristic of an image forming apparatus, said method comprising the steps of: receiving print data, defining a plurality of dot locations, to be printed on a print medium by the image forming apparatus; determining from said plurality of dot locations a dot frequency measure reflecting a number of dots to be printed on at least a part of the print medium; forming a simulated reference image of the print data comprising a plurality of simulated dots corresponding to the plurality of dot locations in the print data; determining a size of the simulated dots in the reference image, said size being inversely related to the dot frequency measure; printing the print data on the print medium using the image forming apparatus to form a printed image; and determining a characteristic of the image forming apparatus based upon a comparison of the printed image and the simulated reference image.