Equation (2) indicates the overall ability level for the ith examinee to answer correctly. Equation (3) indicates the overall difficulty level for the jth item to conquer test-takers. Note that all the indices in person and item facets are calculated using the values without multiplying 100. Through the product of the person’s ability probability with the compensation of the item difficulty probability with a 1/4 power on the product, the BW probability model is revealed in equation (1). Adding a 1/4 power on the product of probabilities in equation (1) is because the magnitude of each BW index is a proportion of all responses (values less than 1) and the product of indices in both equations (2) and (3) with a four-power expression of proportions will result in the product as singularly small.