of 15 or above); in a normative comparison, students' scores are compared against those of a larger group (e.g., scoring above the 25th percentile compared to a national sample of 3rd grade students). Screenings usually occur three times per year (fall, winter, and spring), and the data from these assessments help to guide instruction through the three tiers of the RTI process. This is important not only for identifying students who are having difficulties, but also for identifying possible areas of improvement in the general classroom instruction in the cases where too many students fall below expectations. Because a single universal screening at the beginning of the year can over-identify students who require preventive intervention, the National Research Center on Learning Disabilities recommends that schools also integrate at least five weeks of weekly progress monitoring to identify students who require preventive intervention. Teaching Core curriculum in the classroom should be research-based and field tested. This means, based on evidence from congregating research, that the core curriculum contains all the fundamentals found necessary to efficiently teach reading and has a recognized record of achievement. Such curriculum is to be delivered by "highly qualified" teachers adequately trained to deliver the selected instruction as intended, that is, with fidelity to design. Progress monitoring and tiered service delivery Progress monitoring is a set of assessment procedures for determining the extent to which students are benefiting from classroom instruction and for monitoring effectiveness of curriculum. Curriculum-based measurement (CBM) is often used to collect data on interventions and their effectiveness to determine what works best for an individual student. Additional methods are attempted until students "respond" to the intervention and improve their skills. Students that do not respond, or respond at significantly low rates, may be deemed to have biologically based learning disabilities, rather than simply learning difficulties. Progress monitoring is the scientifically based practice of assessing students' academic performance on a regular basis for three purposes: To determine whether children are profiting appropriately from the instructional program, including the curriculum To build more effective programs for the children who do not benefit To estimate rates of student improvement Three tiers of Scientifically Research-Based Interventions (SRBIs) of increasing intensity incorporate the key components of RTI and help ensure the academic growth and achievement of students. Tier 1 The first tier states that all students receive core classroom instruction that is differentiated and utilizes strategies and materials