Canadians have invested considerable resources in large-scale early and intensive behavioural intervention (EIBI) programs designed to improve the lives of young children with autism spectrum disorder (ASD) and their families. These EIBI programs provide many hours of one-to-one therapy each week to help children to communicate, play, and participate in daily routines. Nova Scotia's EIBI program is unique. It uses a method called Pivotal Response Treatment (PRT) as the main teaching approach. Therapists provide fewer hours of direct therapy than in many other programs. However, treatment is embedded into everyday routines, and parents and childcare centre staff are trained to use strategies throughout children's daily lives. Our research has shown that children in Nova Scotia's program have outcomes that are similar to those of children in more intensive programs. However, in all EIBI programs, outcomes for children with ASD vary widely and are hard to predict. It is important to learn how to improve outcomes for the children who benefit least from current treatment. It is also critical to use resources as efficiently as possible to help the increasing numbers of children being diagnosed with ASD. In the proposed research study, we will develop and test better ways to decide which children benefit most, and least, from the existing Nova Scotia EIBI program. This information will help us to determine how best to adapt treatment to improve outcomes for the children in greatest need. Our long-term goal is to design treatment paths best suited to meet the varying needs of children with ASD and their families while using resources wisely.