Too many children fail to learn how to read proficiently with serious consequences for their overall well-being and long-term success in school. This may be because providing effective instruction is more complex than many of the current models of reading instruction portray; there are Child Characteristic x Instruction (CXI) interactions. Here we present efficacy results for a randomized control field trial of the Individualizing Student Instruction (ISI) intervention, which relies on dynamic system forecasting intervention models to recommend amounts of reading instruction for each student, taking into account CXI interactions that consider his or her vocabulary and reading skills. The study, conducted in seven schools with 25 teachers and 396 first graders, revealed that students in the ISI intervention classrooms demonstrated significantly greater reading skill gains by spring than did students in control classrooms. Plus, they were more likely to receive differentiated reading instruction based on CXI interaction guided recommended amounts than were students in control classrooms. The precision with which students received the recommended amounts of each type of literacy instruction, the distance from recommendation, also predicted reading outcomes. (Contains 7 figures and 6 tables.)