The Dale-Chall Formula was unique because, unlike other formulas that use word-length to assess word difficulty, the Dale-Chall Formula uses a count of ‘hard’ words. The Dale-Chall Formula calculates the US grade level of a text sample based on sentence length and the number of ‘hard’ words. These ‘hard’ words are words that do not appear on a specially designed list of common words familiar to most 4th-grade students.
The original Dale-Chall Formula had a list of 763 non-hard or familiar words. However, the New Dale-Chall Formula, revised by Readability Revisited: The New Dale-Chall Readability Formula, in 1995 expands the list of familiar words to 3000.
The New Dale-Chall Formula is an accurate readability formula for the simple reason that it is based on the use of familiar words, rather than syllable or letter counts. Reading tests show that readers usually find it easier to read, process and recall a passage if they find the words familiar.