This paper investigates the performance of CATSIB (a modified version of the SIBTEST computer program) to assess differential item functioning (DIF) in the context of computerized adaptive testing (CAT). One of the distinguishing features of CATSIB is its theoretically built-in regression correction to control for the Type I error rates when the distributions of the reference and focal groups differ on the intended ability. This phenomenon is also called impact. The Type I error rate of CATSIB with the regression correction (WRC) was compared with that of CATSIB without the regression correction (WORC) to see if the regression correction was indeed effective. Also of interest was the power level of CATSIB after the regression correction. The subtest size was set at 25 items, and sample size, the impact level, and the amount of DIF were varied. Results show that the regression correction was very useful in controlling for the Type I error, CATSIB WORC had inflated observed Type I errors, especially when impact levels were high. The CATSIB WRC had observed Type I error rates very close to the nominal level of 0.05. The power rates of CATSIB WRC were impressive. As expected, the power increased as the sample size increased and as the amount of DIF increased. Even for small samples with high impact rates, power rates were 64% or higher for high DIF levels. For large samples, power rates were over 90% for high DIF levels. (Contains 12 tables and 7 references.) (Author/SLD)