A method for tracking and reporting traffic activity on a web site first comprises storing a web page on a first server coupled to a network. When the web page is requested from a visitor computer, the visitor computer is selected (or not selected) for inclusion within a sample group, where the sample group is only a subset of the total traffic to the web site. A selection indicator is stored on the visitor computer associated with this selection. Data mining code within the web site is operated or not operated depending upon the value of the selection indicator. The data mining code is operated, and the activity of the visitor computer on the web site consequently tracked only if the visitor computer is selected as a member within the sample group, otherwise the data mining code is not operated and no traffic activity is generated.