The use of content validity as the primary assurance of the measurement accuracy for science assessment examinations is questioned. An alternative accuracy measure, item validity, is proposed. Item validity is based on research using qualitative comparisons between (a) student answers to objective items on the examination, (b) clinical interviews with examinees designed to ascertain their knowledge and understanding of the objective examination items, and (c) student answers to essay examination items prepared as an equivalent to the objective examination items. Calculations of item validity are used to show that selected objective items from the science assessment examination overestimated the actual student understanding of science content. Overestimation occurs when a student correctly answers an examination item, but for a reason other than that needed for an understanding of the content in question. There was little evidence that students incorrectly answered the items studied for the wrong reason, resulting in underestimation of the students' knowledge. The equivalent essay items were found to limit the amount of mismeasurement of the students' knowledge. Specific examples are cited and general suggestions are made on how to improve the measurement accuracy of objective examinations.