The construction and validation of a short form of the developmental, diagnostic and dimensional interview.

We aimed to construct and validate a shortened form of the developmental, diagnostic and dimensional interview (3Di), a parent report interview for assessing and diagnosing autistic spectrum disorders (ASDs). Data from 879 children and young people were used. In half of the sample (n = 440) reliability analysis was used to identify 3Di items that best measured each dimension of the autism triad. This informed the construction of a shortened (53 item) 3Di, which was then validated on subjects not used in the reliability analysis (n = 439). This involved comparison with scores from the original 3Di algorithm and, in a subsample (n = 29), with the autism diagnostic interview-revised (ADI-R). Agreement of the new shortened 3Di with the 3Di's original algorithm was excellent in both dimensional and categorical terms. Agreement on caseness (27 out of 29) with the ADI-R was also strong. The new 3Di short version is less than half as long as the original version and outputs very similar scores. It will be useful to clinicians and researchers for obtaining a dimensional autism assessment in less than 45 minutes.