A student at the State University of New York at Binghamton is interested in assessing a new 20-item scale of optimism-pessimism. How should she assess the reliability of this scale? The student is...

A student at the State University of New York at Binghamton is interested in assessing a new 20-item scale of optimism-pessimism. How should she assess the reliability of this scale? The student is also advised by her instructor to measure several different traits using several different methods to demonstrate empirically the convergent and discriminant validity of the new scale. Why did the instructor give this advice?