This method is often used to find related tags, or to cluster the tags.
The co-occurrence coefficient between two tags is the number of resources (photos, links etc.) where both tags are assigned to. To account for the individual frequencies, the co-occurrence value can be normalized.
Symmetric normalization can be done using the Jaccard coefficient.
Asymmetric normalization can be done by
where is the number of times tags and occur together and is the number of times tag occurs.