For implementing equating, the Psychometricians use a 25% overlap of items between the base form and the new test form. These common items constitute the equating link between the two forms. Second, they maintain the average difficulty level (p-value) for both forms in close range. The Psychometricians ensure that the new form’s average difficulty is closer to that of the base form. After a sufficient number of candidates attempt the base test form, the Psychometricians use Livingston’s formula for linear equating—Y* = a (X – c) + d—which uses the mean and standard deviation of both base and new forms and the cut score of the base form to reach the cut score of the new form. These measures help the Art Therapy Credentials Board to ensure that the candidates who pass its examination have the knowledge and understanding expected from a board-certified art therapist.
