The “tiered” model—often illustrated as a triangle with three levels—is central to RTI. Each level represents a grouping of students whose differing needs are met with more intensive instructional approaches, and three points should be kept in mind:

1. The tiers represent actions, not classifications. There is no such thing as a “Tier 2 student”; there are students who are, at a given time and in a given subject, receiving Tier 2 interventions.
2. Core instruction is always retained for students receiving intervention. 
3. The definitions, and even the number, of tiers can vary.

Tier 1, the base or largest level, represents the core instructional program, which all students receive. Ideally, Tier 1 is monitored carefully for effectiveness and at least 80% of students experience success with the instruction provided. Even within the core, however, instruction should be differentiated and personalized as much as possible to produce the best results for higher as well as lower achieving students.

Tier 2 represents a smaller grouping of students who may require additional help—“interventions”—in addition to (though not replacing) the core, to achieve the learning rate necessary to meet academic benchmarks. This tier should represent no more than 10–15% of students. Tier 2 interventions are commonly called “strategic,” “targeted,” or “supplemental.” They may or may not be different from the core, but they are always more.

Tier 3 represents a still smaller group who need even more assistance—“intensive” interventions—to achieve the same goals. This tier is meant to include perhaps 5–10% of students. Tier 3 interventions are generally individualized.