Wesolowsky’s method is manifested in the SCheck13 program. One of its features involves a “Q-Q” plot of results – the plot below was produced by SCheck for the test center A data set:
A Q-Q plot, a quantile-quantile plot, may be used to check on the goodness of a model. The “Calculated Z’s” along the graph’s abscissa correspond to SCheck’s test statistic, Zb, a standardized deviate calculated from the responses of each pair of students (Wesolowsky, 2000). The “Equivalent Normal Z’s” along the ordinate permit an assessment of the degree to which the distribution of the test statistic follows the normal (Gaussian) curve – if the calculated Zb values are normally distributed, the points in the plot will fall on a straight line.