Clickthrough is the rate at which viewers actually click on ad banners and go to the advertiser's site - whether to sign up for something, to make a purchase, or just to find out more. It generally ranges from 1 to 3 percent industry-wide, a calculation arrived at by dividing the gross number of clicks by the gross number of advertising impressions served for a given campaign. The best clickthrough we ever achieved on a campaign was 20 percent, possibly because our banners simulated JavaScript error alert boxes. (No one will ever accuse us of taking the highbrow approach.)