It was introduced in 1953 in Spache's "A new readability formula for primary-grade reading materials," (The Elementary School Journal, 53, 410–413), and has subsequently been revised.
Calculation
The method compares words in a text to a set list of everyday words. The number of words per sentence and the percentage of unfamiliar words determine the reading age.
The original formula was:
The revised formula is:
Further reading
Spache, G. (1953). "A New Readability Formula for Primary-Grade Reading Materials". The Elementary School Journal. 53 (7): 410–13.
doi:
10.1086/458513.
JSTOR998915.
S2CID145135468.
Clarence R. Stone. "Measuring Difficulty of Primary Reading Material: A Constructive Criticism of Spache's Measure." The Elementary School Journal, Vol. 57, No. 1 (Oct. 1956), pp. 36–41