The Style Index tells you if you are writing in clear English. Rather than use a simplistic readability formula of word length and average sentence length, the Style Index measures the plain English problems in your text, including a weighted score for long sentences. In this way, the Style Index gives you an overall score of style based on many writing issues:
The best writers score as low as 10 on the Style Index, equivalent to one style problem for every five or six sentences. Most people score 6 or 7 times as badly as the best writers, averaging two or three style problems in every sentence.