A computer implemented method of generating a validated diagnostic tool for use on a subject by applying artificial intelligence to an instrument for diagnosis of autism or an autism spectrum disorder, the instrument comprising either the Autism Diagnostic Observation Schedule-Generic (ADOS-G) or the Autism Diagnostic Interview-Revised (ADI-R), and wherein the instrument comprises a full set of diagnostic items comprising observed behavior or questions and the diagnostic tool comprises a subset of said full set of diagnostic items, the computer implemented method comprising: on a computer system having a processor and a memory storing a computer program for execution by the processor, the computer program including instructions for: receiving as input answers to the questions or descriptions of the observed behavior coupled with clinical diagnoses from a plurality of previously diagnosed subjects, testing the answers to the questions or descriptions of observed behavior from the instrument and the coupled clinical diagnoses using said artificial intelligence to construct a behavioural classifier to diagnose the autism or the autism spectrum disorder; determining from the testing and said behavioural classifier a most statistically accurate subset of the full set of diagnostic items from the instrument; selecting the subset of the most statistically accurate diagnostic items from the instrument; determining an accuracy of the subset of the most statistically accurate diagnostic items from the instrument by testing the subset of the most statistically accurate diagnostic items from the instrument against an independent source of clinical data from a population of individuals having autism or autism spectrum disorder, wherein the accuracy of the subset of most statistically accurate diagnostic items is over 90%; generating the diagnostic tool for diagnosis of the autism or autism spectrum disorder of the subject, the diagnostic tool comprising the most statistically accurate subset of diagnostic items for use with the behavioural classifier to diagnose the subject. receiving as input answers to the questions or descriptions of the observed behavior coupled with clinical diagnoses from a plurality of previously diagnosed subjects, testing the answers to the questions or descriptions of observed behavior from the instrument and the coupled clinical diagnoses using said artificial intelligence to construct a behavioural classifier to diagnose the autism or the autism spectrum disorder; determining from the testing and said behavioural classifier a most statistically accurate subset of the full set of diagnostic items from the instrument; selecting the subset of the most statistically accurate diagnostic items from the instrument; determining an accuracy of the subset of the most statistically accurate diagnostic items from the instrument by testing the subset of the most statistically accurate diagnostic items from the instrument against an independent source of clinical data from a population of individuals having autism or autism spectrum disorder, wherein the accuracy of the subset of most statistically accurate diagnostic items is over 90%; generating the diagnostic tool for diagnosis of the autism or autism spectrum disorder of the subject, the diagnostic tool comprising the most statistically accurate subset of diagnostic items for use with the behavioural classifier to diagnose the subject.