An extensive list of criteria and book attributes has been developed for each Reading IG level. These criteria were based on an analysis of published books that had been officially leveled using Fountas & Pinnell and Reading Recovery leveling criteria, as well as other factors known to affect text difficulty. Next, both the text of Reading IG books and books assigned official levels by Reading Recovery and Fountas & Pinnell were entered into a computer. Objective criteria--including total word count, high-frequency word count, low-frequency word count, sentence length, word length, number of different words, and other factors--were entered into a mathematical formula. These factors were weighted according to their importance at each level. This analysis yielded a numerical value for each book and ratios such as high- frequency words to total words and different words to total words. In addition to objective measures, subjective factors--such as picture support, concept difficulty, sentence complexity, and more.--were analyzed by trained leveling specialists. These factors also were assigned numerical values and entered into the mathematical formula. Each book was assigned a total numerical value that was used to correlate Reading IG books with other leveling systems. Writing Reading IG books to pre-established criteria, along with further analysis to determine Reading IG correlations to other systems, ensures that Reading IG books are leveled consistently and with a high degree of accuracy.