Case- and form-sensitive letter frequencies in children's picture books

Letter knowledge is a critical preliteracy skill that is essential for school readiness. Interventions designed to develop children's letter knowledge have been based on caregivers' and teachers' picture book reading. Despite the importance of picture books for developing children's letter knowledge, there are no published norms of letter frequencies for picture books. To create case-sensitive letter frequencies for children's picture books, we analyzed a corpus of 100 picture books containing 68,741 words made up of 264,063 lowercase and 14,134 uppercase letters. Analyzing the corpus, we determined that letter frequencies varied by letter and case (p<.001) and thus we created case-sensitive letter frequencies and letter ranks. Comparing uppercase and lowercase letter frequencies, there was a moderate correlation (r=.520). Comparing the children's picture book letter frequencies to adult letter frequencies, there were high correlations between uppercase (r=.815) and lowercase (r=.979) letter frequencies. Further, we extend conceptualizations of letter frequency norms from letter case to letter form. By measuring each letter's features, we determined which letters had the same form across uppercase and lowercase. We used these calculations to create a new set of frequency norms based on the forms of letters. We suggest that the similarities and differences across case should also be accounted for when ranking letter frequencies, especially when considering exposure to uppercase letters. These norms may provide a better representation of the letter forms that children are exposed to and can be used to better understand the foundations of literacy development. (C) 2020 Elsevier Inc. All rights reserved.