The Conversion Rate
http://infocentre.frontend.com/infocentre/articles/conversionrate.html

Farrell, Tom
Frontend Infocentre
2001

Abstract:

Why is usability such an important factor in the success of e-business developments? A key concept in understanding the value of usability is often called the 'conversion rate'. Simply put, it represents the percentage of unique visitors who go on to interact with the site in a pre-defined way. Usually this means make a purchase, but depending on the site in question it could mean registering for more information, placing a bet or opening an account. Conversion rates are usually low. That isn't particularly surprising - they are low in almost any industry, online or offline. What is particularly relevant in the context of usability is the huge benefits that a small change in the rate can offer.