18. The apparatus of claim 17 , the processor configured to: perform, before selecting the similarity audience, the clustering algorithm to update the plurality of clusters using the first plurality of data points and at least some of the second plurality of data points as input.
17. The apparatus of claim 15 , the processor configured to: receive, before receiving the first plurality of data points and for at least one website, a second plurality of data points related to each visitor of a second plurality of visitors to the website, the second plurality of data points comprising at least an identification of the visitor and an interaction of the visitor with the website; and perform a clustering algorithm to generate the plurality of clusters using the second plurality of data points as input before receiving the first plurality of data points.