Equating is a statistical procedure, often used to compare the difficulty of tests which share questions. Equating is used with standardized tests because reusing all questions is considered unacceptable for security reasons, and it is impossible to create tests with different questions but exactly equal difficulty. By comparing performance on equating questions as a group, it is possible not only to compare the difficulty of the tests, but also to standardize scores or percentiles to reflect the same level of performance through separate test forms.

For instance, every SAT exam has a whole 30 minute equating section which reuses old questions but does not count towards the taker's individual score at all. Instead, it is used in the equating procedure. This section may be either verbal or math, and is identical in such a way as to make it impossible to figure out which section is the equating section. (The only legitimate way to determine that would be to have seen the questions on prior exams you had taken and recognized them.)

Equating is complex, and like most statistical procedures, it is powerful but only meaningful when its assumptions are met and it is interpreted correctly. Most importantly, "metric equivalance" is not the same as "meaning equivalance." Basically, it comes down to how you define "hard" or "difficult" in terms of test difficulty. To use an extreme example, it might be perfectly valid to say that the questions "What is the capital of Kenya?" (Nairobi) and "What are the factors of 654321?" (3 and 218607) are metrically equivalant in difficulty for a certain population, but they can hardly be meaningfully equivalent.