A method proposed by McCuaig and Hutchings to assess the internal readability variation of books using the Fry readability formula was empirically tested in an analysis of six elementary basal and ten secondary remedial readers. Each book was treated as a series of consecutive 100-word samples. Means and standard deviations were computed for both sentence length and syllable count measures. Using this information, several different pairs of intervals were constructed for each book using McCuaig's and Hutchings' method. These intervals were designed so that their intersections resulted in 11 different-sized zones, which, according to McCuaig and Hutchings, should have contained from 10 to 95% of the samples within the text. The analysis indicated that this procedure consistently underestimated the readability variation. The procedure, however, can be corrected with one alteration: the probability of a sample lying within the zone formed by the intersection of the two intervals is equal to the product of the separate probabilities of these intervals (the law of independent events). (FL)