Clickthrough rate

Clickthrough rate or CTR is a way of measuring the success of an online advertising campaign.
The CTR for an ad is defined as the number of clicks on an ad divided by the number of times the ad is shown (impressions), expressed as a percentage.
For example, if a banner ad was delivered 100 times (100 impressions) and received one click then the CTR would be 1%.
Banner ad click-through rates have fallen over time; when they first started to appear, it was not uncommon to have rates above five percent. They have fallen since then, currently averaging closer to 0.2 or 0.3 percent. In most cases, a 2% click-through rate would be considered very successful, though the exact number is hotly debated and would vary depending on the situation. The average click-through rate of 3% in the 1990s declined to 0.28% by 2003. Since advertisers typically pay more for a high click-through rate, getting many click-throughs with few purchases is undesirable to advertisers. Similarly, by selecting an appropriate advertising site with high affinity (e.g. a movie magazine for a movie advertisement), the same banner can achieve a substantially higher CTR. Personalized ads, unusual formats, and more obtrusive ads typically have higher click-through rates than standard banner ads, however overly intrusive ads are often avoided by viewers.