Type-Token ratio

The type-token ratio (TTR) is a measure of vocabulary variation within a written text or a person’s speech. The type-token ratios of two real world examples are calculated and interpreted. The type-token ratio is shown to be a helpful measure of lexical variety within a text. source

You can use the Type-Token ratio to evaluate the complexity of a text to improve your search engine ranking and write a text closer to the page that rank higher in the SERP.