# Co-occurrence metrics

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

${\displaystyle {{|t_{i}\cap t_{j}|} \over {|t_{i}|}}}$

where ${\displaystyle |t_{i}\cap t_{j}|}$ is the number of times tags ${\displaystyle t_{i}}$ and ${\displaystyle t_{j}}$ occur together and ${\displaystyle |t_{i}|}$ is the number of times tag ${\displaystyle t_{i}}$ occurs.