Dice coefficient is a similarity measure related to the Jaccard index.
For sets X and Y of keywords used in information retrieval, the coefficient may be defined as:
When taken as a string similarity measure, the coefficient may be calculated for two strings, x and y using bigrams as follows:
where nt is the number of character bigrams found in both strings, nx is the number of bigrams in string x and ny is the number of bigrams in string y. For example, to calculate the similarity between:
We would find the set of bigrams in each word:
Each set has four elements, and the intersection of these two sets has only one element: ht.
Inserting these numbers into the formula, we calculate, s = (2 · 1) / (4 + 4) = 0.25.