After the CAARS 2–ADHD Index items were selected (12 items for Self-Report and 12 items for Observer), raw scores were calculated by summing each set of items. Probability scores were created from the raw scores in a two-step process. First, the distributions of raw scores across age groups within the General Population subset of the Combined Gender Normative Samples and the Combined Gender ADHD Reference Samples were examined. Then the cumulative frequency of each raw score distribution was determined, and a ratio was calculated between the proportion of the ADHD and General Population distributions at or above each raw score. In instances where the cumulative frequency of the given raw score was less than 50%, the proportions were calculated at or below the given raw score; this process allowed the tails of each distribution to be captured. This ratio reflects the probability of obtaining a score that was more likely to have been drawn from the ADHD Reference Sample’s distribution of raw scores, relative to the likelihood of being derived from the General Population’s distribution of raw scores, and it is expressed as a probability score.

Probability scores were calculated separately for each normative age group. Probability scores were hand-smoothed across age group bins to avoid discontinuity between adjacent age groups for corresponding raw scores, as well as to improve interpretability. CAARS 2–ADHD Index probability scores range from 1% to 99%, with a value of 50% indicating that the obtained score is equally likely to occur in the ADHD Reference Sample as in the General Population sample. Guidelines for interpreting the ADHD Index probability score can be found in chapter 4, Interpretation.

The CAARS 2–ADHD Index is a unique Index with a unique score, and evidence of its reliability, validity, and fairness are presented next in this chapter. This evidence is based on the raw score of this measure to facilitate analysis and interpretation where necessary, and on the probability score (where applicable) to evaluate the applied use of this Index.