Embodiments of the present invention relate to identifying website visitors. Initially, a predictor is trained with a set of data of known website visitors to identify a rule with the highest effectiveness score. To do so, each rule in a set of rules is applied to all cookies in the set of data. Based on a selected goal of identifying unknown website visitors, the rule with the highest effectiveness score is identified. To identify a cookie of an unknown website visitor, a cookie representation corresponding to the cookie is identified. The cookie representation represents the cookie as an n-dimensional vector of features and is computed using hit statistics for various aspects of the cookie. Utilizing the cookie representation, a cookie-stitching rule is selected and applied to the cookie. In this way, a website visitor associated with the cookie can be identified.