A, Numbers of study participants, as reflected by registry enrollees, of all ages by US Census tract for Rhode Island and neighboring geographic regions of Massachusetts. B, Variation in rate of ascertainment (i.e., depth of enrollment), as reflected by the number of RI-CART study enrollees in the ASD or ASD-I subgroups relative to the estimated number of cases, age 3 years old to 21 years old and assuming a base-rate prevalence of 1.5%, for each given Census block (see Methods section).