A method for characterizing cross-track spacing variations for a plurality of printer subsystems of an electrophotographic printing system, comprising: printing a first test pattern (300, 310); printing a second test pattern (305, 315) having different sensitivities to cross-track spacing variations of different printer subsystems than said first test pattern; capturing an image of the printed first test pattern (300, 310) to provide a first digitized test pattern including a first array of pixel values; capturing an image of the printed second test pattern (305, 315) to provide a second digitized test pattern including a second array of pixel values; analyzing the first digitized test pattern to determine a first set of parameters (340, 350) that characterize an attribute of the printed first test pattern (300, 310) as a function of cross-track position; analyzing the digitized second digitized test pattern to determine a second set of parameters (345, 355) that characterize an attribute of the printed second test pattern (305, 315) as a function of cross-track position; and using a first defect model to determine an estimated first cross-track spacing variation for a first printer subsystem as a function of the determined first set of parameters (340, 350) and the determined second set of parameters (345, 355), characterised in that determining the first defect model comprises: printing the first (300, 310) and second (305, 315) test patterns at a set of known cross-track spacing variation levels; determining the first and second sets of parameters (340, 345) for each of the known cross-track spacing variation levels; and determining a first mathematical function that defines the first defect model responsive to the known cross-track spacing variation levels and the determined first and second sets of parameters, wherein the first mathematical function predicts the first cross-track spacing variation level for the first printer subsystem as a function of the first and second sets of parameters. printing a first test pattern (300, 310); printing a second test pattern (305, 315) having different sensitivities to cross-track spacing variations of different printer subsystems than said first test pattern; capturing an image of the printed first test pattern (300, 310) to provide a first digitized test pattern including a first array of pixel values; capturing an image of the printed second test pattern (305, 315) to provide a second digitized test pattern including a second array of pixel values; analyzing the first digitized test pattern to determine a first set of parameters (340, 350) that characterize an attribute of the printed first test pattern (300, 310) as a function of cross-track position; analyzing the digitized second digitized test pattern to determine a second set of parameters (345, 355) that characterize an attribute of the printed second test pattern (305, 315) as a function of cross-track position; and using a first defect model to determine an estimated first cross-track spacing variation for a first printer subsystem as a function of the determined first set of parameters (340, 350) and the determined second set of parameters (345, 355), characterised in that determining the first defect model comprises: printing the first (300, 310) and second (305, 315) test patterns at a set of known cross-track spacing variation levels; determining the first and second sets of parameters (340, 345) for each of the known cross-track spacing variation levels; and determining a first mathematical function that defines the first defect model responsive to the known cross-track spacing variation levels and the determined first and second sets of parameters, wherein the first mathematical function predicts the first cross-track spacing variation level for the first printer subsystem as a function of the first and second sets of parameters. printing the first (300, 310) and second (305, 315) test patterns at a set of known cross-track spacing variation levels; determining the first and second sets of parameters (340, 345) for each of the known cross-track spacing variation levels; and determining a first mathematical function that defines the first defect model responsive to the known cross-track spacing variation levels and the determined first and second sets of parameters, wherein the first mathematical function predicts the first cross-track spacing variation level for the first printer subsystem as a function of the first and second sets of parameters.