The main function of readability tests is to give you a quick assessment about the density of your writing. Readability tests cannot tell you how easily a reader can understand the information in the text.
You can perform readability tests manually by counting and doing a mathematical calculation, or by using word-processing software. There are several popular readability indexes, for example:
- Gunning Fog Index
- Flesch Reading Ease Scale
- Flesch-Kincaid Grade Index
8.4.1. Gunning Fog Index
To calculate the Gunning Fog Index of a passage, do the following:
- Count the number of words in the paragraph: W
- Count the number of sentences in the paragraph: S
- Count the number of words of three syllables or more: T
- Apply the following formula: [W/S + T/(Wx100)] x 0.4
8.4.2. Flesch Reading Ease Scale
Calculate the readability of a passage on the Flesch Reading Ease Scale in the following way:
- Calculate the average sentence length: L
- Calculate the average number of syllables per word: N
- Calculate the score between 0 and 00%.
The higher the score, the easier the text is to understand. Aim to maximize the score. The Flesch Reading Ease Scale measures readability as follows:
|100||Very easy to read.||Average sentence length is 12 words or less. No words of more than two syllables.|
|65||Plain English.||Average sentence length is 15 to 20 words. Average word has two syllables.|
|0||Extremely difficult to read.||Average sentence length is 37 words. Average word has more than two syllables.|
Click on the following link to find out more about the Flesch Readability Test. In this link, Rudolf Flesch, the developer of the test, provides some background information.
8.4.3. Flesch-Kincaid Index
This index computes readability based on the average number of syllables per word and the average number of words per sentence. The score indicates the number of years of combined primary and secondary education that a reader needs to understand a text. For example, a score of 8.0 means that you can understand the text after eight years of combined primary and secondary education. Standard writing achieves around 7.0 to 8.0 on the Flesch-Kincaid Index.
Click on the following link to find out more about the Flesch-Kincaid Index and other readability tests. Although this web page deals with textbooks, you can also apply the concepts to technical documentation.