Many research had shown person fit indices might be influenced by the factor of test length on their detection rates of aberrant responses. The purpose of this study was to examine test length effects on the BW aberrance indices. Three conditions were designed in this study: test length (K, including 25, 50,100, and 200 items), ability ratio (T/K, defined as the total person score divided by test length K), and error ratio (E/K, defined as the number of errors within ability level divided by test length). Four 100-person times varying-item data matrices (100x25, 100x50, 100x100, and 100x200) were randomly generated and permuted 500 times for each data matrix through 20 repeats. Results showed that after partialling out the factors of E/K and T/K, the effect of test length on the association between the two indices was very slight. In nonlinear regression analyses, E/K and T/K can predict more than 76 and 73 percent of the variances of the B index and that of the W index, respectively, but test length with both very slight contributions on them. Furthermore, a very good model fit generated from SEM analyses also showed the effect of test length on the B and W indices were very tiny. All these pieces of evidence endorsed the B and W indices were robust with test length.
