Studies By Forrester Research Estimate Several Costs Of Bad Design Product The Two Most Striking Are

usability.gov, Mar 10, 2005

Why Is Usability Important?
Research by User Interface Engineering, Inc., shows that people cannot find the information they seek on Web sites about 60% of the time. This can lead to wasted time, reduced productivity, increased frustration, and loss of repeat visits and money.

"There are about 43 million Web sites, and no one knows which ones are usable. The best sites we've found are usable only 42 percent of the time, and none that we have studied are usable a majority of the time ...."
Studies by Forrester Research estimate several costs of bad site design. The two most striking are:
Losing approximately 50% of the potential sales from a site as people can't find what they need

Losing repeat visits from 40% of the users who do not return to a site when their first visit resulted in a negative experience
 
"Studies of user behavior on the Web find a low tolerance for difficult designs or slow sites. People don't want to wait. And they don't want to learn how to use a home page. There's no such thing as a training class or a manual for a Web site. People have to be able to grasp the functioning of the site immediately after scanning the home page — for a few seconds at most."

What Is the Difference Between Usability Engineering and Usability Testing?

Usability engineering is a methodical approach to producing a Web site or any user interface. It is a practical and systematic way to deliver a product that works for users. Usability engineering involves several methods, each applied at appropriate times, including gathering requirements, developing and testing prototypes, evaluating design alternatives, analyzing usability problems, proposing solutions, and testing a site (or other interface) with users.

Usability testing is part of the process of usability engineering. Usability testing includes a range of methods for having users try out a site (or other system). In a typical usability test, users perform a variety of tasks with a prototype (or other system) while observers record notes on what each user does and says. Typical tests are conducted with one user at a time or two users working together. Testing may include collecting data on the paths users take to do tasks, the errors they make, when and where they are confused or frustrated, how fast they do a task, whether they succeed in doing the task, and how satisfied they are with the experience. The goal of most usability testing is to uncover any problems that users may encounter so those problems can be fixed.