Simple sum procedure of the conditional PDF approach (plausiblity of distractor function) combined with the normal approach method was applied for estimating the plausibility functions of the distractors of the Level II vocabulary subtest items of the Iowa Tests of Basic Skills. In so doing, the normal ogive model was adopted for the correct answers of those items, and those items were used as the substitute for the old test. The subjects were 2,364 fourth to ninth grade students who took the Level II tests. Results indicated the existence of informative distractors for certain test items, and it was confirmed that most items fit the informative distractor model (stating that the incorrect chosen distractor does provide information) rather than the equivalent distractor model. The model validation study indicated that, for most items, the normal ogive model was suitable for their correct answers. (Extensive graphs and data tables are included). (Author/GDC)