1. A method comprising: at one or more computing devices comprising one or more processors and memory storing one or more computer programs executed to perform the method, performing operations comprising: obtaining a first plurality of content items clustered into a first cluster, the first cluster associated with a first numeric representation representing one or more similarities identified among at least two content items of the first plurality of content items; obtaining a second plurality of content items clustered into a second cluster, the second cluster associated with a second numeric representation representing one or more similarities identified among at least two content items of the second plurality of content items; determining a cluster link density based at least upon the first numeric representation and the second numeric representation, the cluster link density representing one or more similarities identified among the first plurality of content items and the second plurality of content items; and based on the cluster link density, forming a merged cluster by merging the first cluster and the second cluster.