The Origins Of The Strain Index

By Nirmaldasan


A flash of inspiration in early 2005 resulted in the Strain Index and the Character-Count Index. On the advice of the readability expert William DuBay, I tested them on 62 graded passages drawn from two books (1). The Pearson’s product moment correlations were 0.8481 for the Strain Index and 0.7106 for the Character-Count Index.

I first published ‘Character-Count Formula For Readable Writing’ in Journalism Online, July 2005. But I subjected the Strain Index to further tests. Under the guidance of Dr. Nirmal Selvamony, I submitted my M.Phil dissertation titled ‘A Quantitative Analysis Of Media Language’ (December 2006, Madurai Kamaraj University), in which I was able to show that the Strain Index is an alternative to the Fog Index of Robert Gunning. Subsequently I wrote an article titled ‘Strain Index: A New Readability Formula’ (July 2007) to popularise it — a formula that DuBay called ‘remarkably simple’.

It may be recorded here that the general form of the Strain Index is the number of syllables per sentence multiplied by 0.3 to match texts with years of schooling. Its original and popular form, however, is S3 / 10; S3 is the number of syllables in three sentences.

But the origins of the Strain Index, though I did not know in 2005, may be traced to Lucius Adelno Sherman’s The Analytics Of Literature: A Manual For The Objective Study Of English Prose And Poetry (1893) and Harry D. Kitson’s The Mind Of The Buyer: A Psychology Of Selling (1921). Sherman calculated the average words per sentence (AWS) in literary texts of different periods. Kitson calculated the ASW (average syllables per word) as well as the AWS of two newspapers. A few decades later, these important measures of readability — the AWS and the ASW — resulted in Rudolph Flesch’s Reading Ease formula (2).       

Dr. William S. Gray and Bernice Leary, in their book titled What Makes A Book Readable (1935), anticipated the Strain Index by calculating the number of syllables per sentence — but chose the AWS as its correlation with text difficulty was better. They missed something that Irving Fang and Harry McLaughlin didn’t. Fang’s Easy Listening Formula (1966-67) is the number of exsyls per sentence; exsyls are syllables more than one per word. McLaughlin, while formulating the Smog Grading Method, says that AWS and ASW should not be added but multiplied. He offers this insight: “Obviously, you must measure word length and sentence length separately if you are going to add the two measures together. But you achieve the equivalent of multiplying the two measures if you simply count out a fixed arbitrary number of sentences and then count, say, the number of syllables within those sentences.” (3)  

Multiplying the AWS and the ASW is the same as counting the number of syllables per sentence. Thus the single Strain Index variable has the predictive power of the AWS and the ASW. So I recommend it, especially its popular form S3 / 10, to measure text difficulty. The numerator owes its origin to so many sources recorded here. But what about its denominator that makes the Strain Index easy to use? I owe that to the FORCAST formula of Patrick FORd, John CAylor and Thomas STicht. With such a history, the ‘remarkably simple’ Strain Index should recommend itself to all.


1. Readability Revisited: The New Dale-Chall Readability Formula (1995) by Jeanne Chall and Edgar Dale. Qualitative Assessment of Text Difficulty, A Practical Guide for Teachers and Writers by Jeanne S. Chall, Glenda L. Bissex, Sue S. Conard, and Susan Harris-Sharples (1996). Both published by Brookline Books, Cambridge.

2. Unlocking Language: The Classic Readability Studies, editorial commentary by William H. Dubay (2007). Published by Impact Information, California.  

3. ‘Smog Grading — A New Readability Formula’: G. Harry McLaughlin, Journal Of Reading, May 1969.


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