Rasch's Model, based on the assumption of equal item discriminating powers, was measured by the probability that the deviation of the data from the expectation was more extreme than the observed deviation. The model was found to fit a number of test items of each test - Maths, Geography, Biology, English. The index of item discrimination was taken as the slope of the item characteristic curve at the point where the probability of correct response was 0.50, and the discrimination indices of items that fit the model were found to be unequal lying in the range of 0.50 - 1.25.