Reading Age (RA) and Reading Grade (RG) are measures of literacy in the U.K. and the U.S. respectively. Both are related concepts tied by this conversion formula: RA = RG + 5. The Schonell Reading Test consisting of a list of 100 words from the monosyllabic ‘tree’ to the polysyllabic ‘idiosyncracy’ helps evaluate the RA as well as the RG of a child. If C is the number of words correctly pronounced by a child, then RA = (C/10) + 5; and RG = C/10. If RA is 13, then RG is 8.
Martin Cutts and William DuBay are agreed on the average reading age/grade. Here is the 10th guideline of Martin Cutts: “Remember the average reading age of the population: about 13 years.” (Oxford Guide To Plain English, 3rd edition, 2009). And William DuBay states: “The average adult in the U.S. reads at the middle-school level, roughly at the 8th grade level.” (Smart Language, 2007). This means that the average child will be able to pronounce correctly about 80 of the words in the Schonell Reading Test.
In two of my articles ‘The Average Sentence Length’ and ‘Syllable, Word and Sentence Length’, I have come to the conclusion that the average sentence length has 17 words, 29 syllables and 87 letters. And now I see that the average reading grade is connected to the average sentence length. Let me demonstrate with three of my readability formulae: HGL (Higher Grade Level), CCI (Character-Count Index) and SI (Strain Index).
HGL = (W4/10) to (W5/10); W4 and W5 are the number of words in four and five sentences respectively. Since the average sentence has 17 words, HGL = 6.8 to 8.5.
CCI = C1/10; C1 is the number of characters (no spaces) in a sentence. Since the average sentence has 87 letters, CCI = 8.7.
SI = S3/10; S3 is the number of syllables in three sentences. Since the average sentence has 29 syllables, SI = 8.7.
The HGL, the CCI and the SI tell us that Cutts and Dubay are right about the average reading grade of the population. Viewed differently, I would like to say that the average sentence length and the average reading grade confirm the validity and the reliability of the HGL, the CCI and the SI.