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A better readability formula : StyleWriter's Bog Index

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StyleWriter's Bog Index is a measure that improves on the standard readability formulas.  Its key feature is to measure readability with a graded wordlist rather than simply using a syllable count, or word length.  It then rates the style and readability of your document according to document type – the Writing Task – and the Target Audience. The Bog Index has three distinct parts, Sentence Bog, Word Bog and Pep.  Bog is anything that detracts from easy reading – it bogs your reader down.  Pep is anything that makes your writing easier to read and more interesting – it peps up your writing.

Bog Index = Sentence Bog + Word BogPep

Sentence Bog deals with the problem of sentence length.  StyleWriter takes the Average Sentence Length for the document, squares it, then divides the result by the Long Sentence Limit for the chosen Writing Task.  This reflects the fact that some Writing Tasks demand shorter sentences.

Sentence Bog = (Average Sentence Length)
Long Sentence Limit

Note: An Average Sentence Length around half the Long Sentence Limit will give a Sentence Bog of around half the Average Sentence Length, which is good.
  • Increasing the Average Sentence Length by 40% will double the Sentence Bog.
  • Reducing the Average Sentence Length by 30% will halve the Sentence Bog.

Word Bog – the second part of the Bog Index, is the crucial measure of word difficulty. Unlike traditional readability formulas, StyleWriter's Bog Index measures:
  • Word difficulty (rather than word length)
  • Abbreviations and acronyms
  • Wordiness
  • Passive verbs
  • Style issues

StyleWriter assigns a Bog value to each of these and expresses the result as the amount found in 250 words of your document.

Word Bog = (Style Problems + Heavy Words + Abbreviations + Specialist) X 250
Number of Words

Pep counts the features in your document that are the hallmarks of good writing.  Pep reduces the Bog Index because it makes the reader's job easier and more enjoyable – even overcoming some of the effects of Bog

StyleWriter's Pep includes:
  • Names
  • Interesting words
  • Conversational expressions, including:
  • Personal pronouns
  • Contractions
  • Direct questions (1 to 10 words)
  • Variation in sentence lengths

StyleWriter assigns a Pep value to each of these features and expresses the result as the amount found in 25 words of your document (one tenth of the effect from Bog).

Pep = (Names + Interest Words + Conversational) x 25 + Sentence Variety
Number of Words


Sentence Variety = Standard Deviation x 10
Average Sentence Length

Note: Standard Deviation is a statistical measure of the variability in a set of numbers – in this case the sentence lengths in your document. If a document contains lots of sentences of similar length, the Sentence Variety would be zero or one.  For most writing, the standard deviation will be about half the Average Sentence Length, making a Sentence Variety of around four or five.  You can increase Sentence Variety by including some short sentences in your document, but it can never be more than ten.

The Bog Index brings all these features into its formula to give a comprehensive measure of writing style.  You can see how each component contributes to your document's Bog Index by expanding the Bog Index entry in the Statistical Summary.

Adjusting the Bog Index by Writing Task and Audience

Good clear writing differs depending on the writing task and the audience.  A medical paper read by scientists naturally uses more difficult words than a speech to the public.  StyleWriter's Bog Index adjusts its score and rating depending on the writing task and audience you choose from the drop-down menus on the Text screen. StyleWriter lowers the Bog Index penalty for long sentences for different writing tasks and its word score depending on the audience.  There's no drop in the word score if you are writing to the public.  If you choose In-house Audience, the program does not penalize you as heavily for using abbreviations and acronyms.  If you choose Specialist Audience, the program does not penalize you so heavily for using abbreviations and acronyms or specialist words.

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