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© 2010 Ugur Akinci
Readability measures how easy to read and understand a document is, assuming the readers are identical in understanding of the subject matter. Readability as a topic was pretty much invented single-handedly by Rudolph Flesch back in the 1940s. Today one of the most popular readability formulas carries his name.
Here are the three better-known readability formulas to measures the difficulty level of your technical document.
1) Flesch Reading Ease Index
READING EASE = 206.835 – (1.015ASL + 0.846ASW)
ASL is the Average Sentence Length and ASW stands for Average Number of Syllables per 100 words.
The Flesch score varies between 0 (zero) and 100. A score of over 90 is interpreted as a “very easy to read” text. Lower the score, harder is the text to read and understand. Text with Flesch score 0-30 would be “very hard” to read.
2) Flesch-Kinkaid Readability Index
GRADE LEVEL (GL) of Difficulty = (0.39NWS + 11.80NSW) – 15.59
NWS is the Average Number of Words in a sentence and NSW is the Average Number of Syllables in a word.
The result of the formula yields the grade level that the reader is expected to have in order to understand the text in question comfortably.
3) Gunning-Fog Readability Index
FOG Index = 0.4 (ASL + TSW)
ASL is the Average Number of Words in a sentence and TSW is the Number of Trisyllabic Words per 100 words.
The trick in counting TSW is that proper words, compound nouns, and verb conjugations do not count. For example “determination” is a trisyllabic word but “opening” is not.
Here is a short text sample and how it fares on these three different indexes:
“A technical specification may be developed privately, for example by a corporation, regulatory body, or military organization, or it may be developed by standards organizations which often have more diverse input and usually develop voluntary standards. (Voluntary standards may become mandatory if adopted by a government or business contract.)
Use the Online Calculators
You can calculate the index values the manual way or use this wonderful online calculator:
When we feed the above sample text, this is the readability value we get:
Flesch-Kincaid Grade level: 17.
Flesch Reading Ease score: 6.
This means to understand this text easily what needs to have almost Graduate Level education. And the Reading Ease score 6 again shows what a tough piece of text this is since a score of under 10 usually denotes hard-to-understand legal documents.
We’ll use another calculator (http://simbon.madpage.com/Fog/) to calculate the Gunning-Fog index. The result is a grade level of 17.4 which again points to a Graduate Level college education.
And lastly, check this MULTIPLE INDEX CALCULATOR out: http://www.addedbytes.com/code/readability-score/