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The Clarity Index

A formula to help you write more clearly

At the start of my career I spent a couple of years working for Deloittes, the big consultancy firm.  In that job, skills training was regarded as very important, and one of the best programmes we were sent on was a two day course on Writing Skills. 

The tagline of that writing course was:  "You should write to EXPRESS, not to IMPRESS".

As part of the course we were taught a neat little formula for working out how clear your writing is.  It's known as the Clarity Index, and it's based on an older idea known as the Fog Factor.

The idea is to take a sample of, say, 200 words from the middle of a piece of your writing, and then apply the following formula to work out its clarity:

(1)  Work out the average sentence length, by dividing the number of words by the number of full stops and question marks.

(2)  Work out the percentage of words that have three or more syllables.  (To count the syllables in a word, say the word with your mouth closed and count the number of "mm"s.  So for example the word 'manipulative' is "mm-mm-mm-mm-mm", which is five syllables.)

(3) Add (1) and (2) together to get your clarity index.

The lower the total the better.  Broadly speaking, the levels are this:


      Below 25                   Primary school children ('The Cat sat on the Mat').           

      25 - 35                       Older children and the general public

      35 - 45                       A highly educated/academic audience

      45+                            Any audience to whom you don't want to convey useful information

On the course we tested this out on different newspapers.  Articles in The Guardian were typically in the 35-40 range.  Articles in The Sun were more typically around 30 - but interestingly, so were those in The Economist.  It was a revelation to me that articles in The Economist are deliberately edited to make them punchy and easy to follow.

I was reminded of this recently when I read a report that summarised a CPD session that I had taken part in.  It included the following paragraph:

Our esteemed panel of speakers delivered insightful presentations on diverse topics, including the utilization of narratives to facilitate mathematical comprehension, the historical context of mathematical principles, and the integration of technology to augment financial literacy.

I make the Clarity Index of that paragraph 80.7 *!!  

That huge score is mainly down to all those multi-syllable words.  Instead of saying "the utilisation of narratives to facilitate mathematical comprehension" they could perhaps have said; "how stories can help with understanding maths".

Long words make things harder to understand, because the brain has to stop and process the words.  I think of this when I hear some of the vocabulary that has entered primary schools in recent years, including  'subtrahend', 'subitising', 'minuend', 'fronted adverbials' and 'manipulatives'.  Calling something a 'manipulative' rather than calling it (say) a 'prop', has consequences. 

In the words of George Orwell: Never use a long word when a short word will do.

If you'd like to check your own clarity index, there are plenty of apps online, for example this one:


* 16 of the 35 words in that one-sentence paragraph (45.7%) have 3+ syllables, so the CI is 35 + 45.7 = 80.7.  Strictly speaking I should have taken a larger sample of words, because long sentences like this one can be balanced with shorter ones.  However, I did check the clarity of the whole report, and it was still above 70.