Severity scores were created by dividing the pool of assessments from individuals with ASD into narrowly defined age and language cells, and standardizing raw total scores from the revised algorithms (Gotham et al., 2007 ) within these cells. In order to maximize the number of cases available for standardization, assessments missing data from any one item from either the Social Affect (SA) or Restricted Repetitive Behavior (RRB) domains of the revised ADOS algorithms were retained by adding to the domain total an average item score from that participant’s existing domain data. The ASD sample alone was used for raw total standardization: this included all assessments corresponding to a best estimate diagnosis of autism or ASD, as well as data from 13 individuals who had ADOS data with a contemporaneous nonspectrum diagnosis but who were later diagnosed with ASD. This subsample (N=1807 assessments from 1118 individuals) was separated into groups based on the five revised algorithms used with children: Module 1 No Words, Module 1 Some Words, Module 2 Younger than 5; Module 2 Age 5 and Older; and Module 3. Within each of these five developmental cells, distributions of summed Social Affect and Restricted Repetitive Behaviors totals were generated separately for every one-year age group between 2 and 16 years; these age cells were collapsed when possible in order to create the fewest number of age- and language-level-determined ‘calibration cells’ with similar raw total score distributions. Younger age cells were purposely kept distinct to anticipate developmental changes and more frequent assessments in young children as they transition from toddlerhood to preschool to school programs. Age cells with similar distributions were collapsed only within the same algorithm. Eighteen calibration cells resulted (see Figure 1 ).
Within each of these 18 cells, raw ADOS totals were mapped onto a 10-point severity metric. After considering a variety of approaches, severity scores 1–3 were set so as to represent the distribution of raw scores receiving a nonspectrum ADOS classification within that calibration cell, severity scores 4–5 represented ASD-classification ADOS totals, and 6–10 represented raw totals receiving an autism classification within that cell. ADOS classification thresholds were determined by the revised algorithm relevant to each calibration cell. The range of raw totals corresponding to each point on the severity metric was determined by the percentiles of available data associated with each severity point within a classification range. Lower severity scores are associated with less autism impairment.Table 2 shows the raw score range corresponding to each severity point within each calibration cell.
Within each of these 18 cells, raw ADOS totals were mapped onto a 10-point severity metric. After considering a variety of approaches, severity scores 1–3 were set so as to represent the distribution of raw scores receiving a nonspectrum ADOS classification within that calibration cell, severity scores 4–5 represented ASD-classification ADOS totals, and 6–10 represented raw totals receiving an autism classification within that cell. ADOS classification thresholds were determined by the revised algorithm relevant to each calibration cell. The range of raw totals corresponding to each point on the severity metric was determined by the percentiles of available data associated with each severity point within a classification range. Lower severity scores are associated with less autism impairment.