The Fog Index: A Tool For Checking The Readability Of Your Writing

lost in the fog

Foggy Writing

Foggy writing is difficult to understand.  The reader can’t see where they are going nor where they have been.  Unfortunately, writers frequently struggle to  realise when their writing is unclear. This can lead to extremely long and unreadable sentences that only the author can understand. Fortunately, writers have a some  useful tools at their disposal to help them avoid foggy writing. One of them is the Fog Index which generates an instant readability score for writing in English.

How Does it Work

Readability is subjective and naturally impossible  to measure perfectly. Nonetheless, the  Fog index has  broken it down into formula.  It measures the sentence length and the number of syllables per word  to generate a readability rating. The higher the rating the more difficult it is to read and vice versa.

The score indicates the number of years of education a reader requires to understand a text. For example the New York Times averages a score of 12, indicating it is suitable to read for all those who have graduated high school. Most professional texts average between 10 and 15, while even the most academic of texts rarely record scores higher than 18.  Therefore, what represents a “suitable” Fog Index rating depends upon the texts  intended audience.

Using the Fog Index 

The system is not perfect and therefore a writer should only use it as a guide.  For instance, taking the number of syllables as a proxy for difficult words causes the Fog Index to regard “asparagus” as difficult as “ontology“. Moreover,  poor short sentences can be as difficult to understand as  well- constructed long sentences. The Fog index does not take this into account.

In practice this means that for an academic paper if the score is below 10 then a writer should re-read their text and check if they have oversimplified. If it is over 15 a writer may consider breaking up some of their sentences. Ultimately though, the writer must use their judgement on whether changes are needed. To generate the most accurate Index scores, the text should be put into the program at roughly 100 words (or one paragraph) at a time.

Gunning Fog Index generator is available  here.

This blog post has a score of around 12. That seems more or less right for my intended audience of bachelor and masters students. However   I will read it over again anyway; I don’t want to write a foggy article about how to avoid foggy writing. 


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