In the 1920s, the Scientific Movement in education looked for tests to measure students' achievement to aid in curriculum development. Teachers and educators had long known that readers, especially beginning readers, should have reading material that closely matched their ability, to help improve their reading skill. University-based psychologists did much of the early research, which was taken up later by publishers of textbooks.
Educational psychologist Edward Thorndike of Columbia University noted that in Russia and Germany teachers were using word frequency counts to match books with students. Word skill was the best sign of intellectual development and the strongest predictor of reading ease. In 1921, Thorndike published his Teachers Word Book, which contained the frequencies of 10,000 words. It made it easier for teachers to choose books matching the reading skills of their class. It also laid down the basis for all research to come on reading ease.
Until computers came along, word frequency lists were the best aids for grading the reading ease of texts. In 1981 the World Book Encyclopedia listed the grade levels of 44,000 words.