Click-through rate (CTR) is the ratio of users who click on a specific link to the number of total users who view a page, email, or advertisement. It is commonly used to measure the success of an online advertising campaign for a particular website as well as the effectiveness of email campaigns. Click-through rates for ad campaigns vary tremendously. The first online display ad shown for AT&T on the website HotWired in 1994, had a 44% click-through rate. With time, the overall rate of user's clicks on webpage banner ads has decreased.