children cannot become fully literate until they develop fast recognition of words and “accurate production of words in writing”

Gunning 'FOG' Readability Test

Select samples of 100 words, normally three such samples. (i)
Calculate L, the average sentence length (number of words ÷ number of sentences).
Estimate the number of sentences to the nearest tenth, where necessary. (ii)
In each sample, count the number of words with 3 or more syllables.
Find N, the average number of these words per sample. Then the grade level needed to understand the material = (L + N) × 0.4
So the Reading Age = [(L + N) × 0.4 ] + 5 years. This 'FOG' measure is suitable for secondary and older primary age groups.

Fry Readability Graphs

Select samples of 100 words.

(i) Find y, the average number of sentences per 100-word passage (calculating to the nearest tenth).

(ii) Find x, the average number of syllables per 100-word sample.

Then use the Fry graph (below) to determine the reading age, in years.

This test is suitable for all ages, from infant to upper secondary.

The curve represents normal texts. Points below the curve imply longer than average sentence lengths. Points above the curve represent text with a more difficult vocabulary (as in school science texts).

Powers-Sumner-Kearl Formula
This is the only one of the formulae suitable for primary age books.

Select samples of 100 words.

(i) Calculate L, the average sentence length (number of words ÷ number of sentences). Estimate the number of sentences to the nearest tenth, where necessary.

Count N, the number of syllables per 100 words.
Then grade level = ( L × 0.0778 ) + ( N × 0.0455 ) - 2.2029