A new indicator of item difficulty, which identifies effectiveness ranges, overcomes the limitations of other item difficulty indexes in describing the difficulty of an item or a test as a whole and in aiding the selection of appropriate ability level items for a test. There are three common uses of the term "item difficulty": (1) the probability of a correct answer to a test item, based on the perception of the person being tested; (2) a group summary statistic that tells what proportion of a group can answer an item correctly; and (3) a measure of a location along a difficulty continuum based on item response theory (IRT) methodology. Using formulas based on IRT methodology, the new indicator measures the range of the score scale in which the item was effective and the point where the item is most informative. Unlike other methods, it is sample independent and can be used more directly for test construction by representing each item as a block, then selecting and stacking these blocks to create the distribution of test items on an ability scale. (ES)