Assessment exercises (items) in three different formats--multiple-choice with an "I don't know" (IDK) option, multiple-choice without the IDK, and open-ended--were placed at the beginning, middle and end of 45-minute assessment packages (instruments). A balanced incomplete blocks analysis of variance was computed to determine the biasing effects of position or format on the national percent correct. Format was found to create a bias, but position did not, except for 9-year-old respondents. (Author) Aspect of National Assessment (NAEP) dealt with in this document: Procedures (Exercise Development).