Abstract: A Web advertising measurement system that correlates the number of impressions of Web advertisements with post-impression transactional activity to measure the effectiveness of the advertisements. When a user clicks on a banner advertisement, an impression is established and the user's identification is recorded. Then, when the user undertakes post-impression transactional activity such as downloading software related to the advertisement, ordering products and services related to the advertisement, and so on, the transactional activity along with the user's identification is recorded. Based on the user identifications, the number of impressions associated with the advertisements are correlated to the post-impression transactional activity as a measure of effectiveness of each advertisement.