Usability Professionals, salary survey. The Usability Professionals’ Association (UPA) supports people who research, design, and evaluate the user experience of products and services. The UPA conducted a salary survey of its members (UPA Salary Survey, Aug. 18, 2009). One of the report’s authors, Jeff Sauro, investigated how much having a PhD affects salaries in this profession and discussed his analysis on the blog www.measuringusability.com. Sauro fit a first-order multiple regression model for salary (y, in dollars) as a function of years of experience (x1), PhD status

(x2 = 1 if PhD, 0 if not), and manager status (x3 = 1 if manager, 0 if not). The following prediction equation was obtained:

yn = 52,484 + 2,941x1 + 16,880x2 + 11,108x3

a. Predict the salary of a UPA member with 10 years of experience who does not have a PhD but is a manager.

b. Predict the salary of a UPA member with 10 years of experience who does have a PhD but is not a manager.

c. Why is a 95% prediction interval preferred over the predicted values given in parts a and b?