Upon the selection of the Conners 4–ADHD Index item content, raw scores were calculated by summing the final sets of 12 items, with the additional weighting applied to three items for Teacher and two items for Self-Report. Probability scores were created from the raw scores by first examining 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. The cumulative frequency of each raw score distribution was determined, and a ratio was then calculated between the proportion of the ADHD and General Population distributions at or above each raw score (or, 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, to capture the tails of each distribution). 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.

The ratio, or the contrast in likelihood of a raw score being more probable within the ADHD Reference Sample’s distribution than the General Population’s distribution, is expressed as a probability score. Probability scores are calculated separately for each normative age group. Probability scores were hand-smoothed to avoid discontinuity between adjacent ages for corresponding raw scores and to improve interpretability. The probability score ranges from 1% to 99%, with a value of 50% indicating that the obtained score on the Conners 4–ADHD Index is equally likely to occur in the ADHD Reference Sample as it is in the General Population sample. Guidelines for interpreting the ADHD Index probability score can be found in chapter 4, Interpretation.

The Conners 4–ADHD Index is a unique index with a unique score, and evidence of its reliability, validity, and fairness are presented within this chapter based on the raw score of this measure to facilitate analysis and interpretation where necessary, and on the probability score (where applicable) to understand the applied use of this index.