The 3 best approaches for IRT equating

If you are delivering high stakes tests in linear forms – or piloting a bank for CAT/LOFT – you are faced with the issue of how to equate the forms together. That is, how can we defensibly translate a score on Form A to a score on Form B? While the concept is simple, the methodology can be complex, and there is an entire area of psychometric research devoted to this topic. There are a number of ways to approach this issue, and IRT equating is the strongest.

Why do we need equating?

The need is obvious: to adjust for differences in difficulty to ensure that all examinees receive a fair score on a stable scale. Suppose you take Form A and get s score of 72/100 while your friend takes Form B and gets a score of 74/100. Is your friend smarter than you, or did his form happen to have easier questions? Well, if the test designers built in some overlap, we can answer this question empirically.

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