What is standard deviation for sentence length?

One indicator of a lively prose style is varied sentence length. Same-length sentences can put the reader to sleep. Standard deviation (SD) is a quantitative indicator of sentence length variety.

The preceding paragraph, for example, is quite boring to read, despite its potentially interesting content. It has average sentence length of 10 words and standard deviation of only 1.5 words. Low SD = little variation in sentence length.

As a rough guide, information text sentence lengths are similar, about SD=8 in the case of the English demo text, more typcally about SD=6, while fiction tends to be more varied, SD=9 and up.

Use this routine to compare texts on this point (different languages, genres, purposes, etc.)