Fidelity! Fidelity! Fidelity! -- What About Dosage?

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    One of the critical elements of any RTI program is having in place a set of procedures to ensure that the evidence-based practices used at the various tiers are being implemented with high fidelity. In the absence of regular checks on fidelity of implementation, we won’t know if the interventions are being taught with integrity. In order to make sound judgments about a student’s responsiveness to an intervention, we must be assured that the intervention is being taught in a way that is consistent with its initial design. If we don’t know if the intervention is being taught correctly, inappropriate decisions may be made about student performance — i.e., we may attribute poor performance to the student when it may be due primarily to ineffective implementation. A great deal has been written in the professional literature about the importance of doing fidelity checks and ways to do them.

    While this attention on fidelity is appropriate, it is not sufficient! It is equally important to know that interventions are being taught in the right dosage.


    Dosage is defined as the amount of something to be administered at one time and the intervals at which it should be administered for a specified period of time.  In short, an intervention may be delivered correctly every time that it is taught (i.e., the intervention has high rates of fidelity), but if it is only taught two days a week when it should be taught every day of the week, outcomes may be compromised because dosage is not being carefully monitored.

    Unfortunately, dosage has been ignored or at least not carefully measured in the majority of studies designed to improve reading comprehension in adolescents. There is considerable variability in the application of reading comprehension interventions for students with LD.  Instructional dosage can be thought of as being composed of at least four interconnected factors: (a) group size, (b) instructional period, (c) frequency, and (d) duration.  Group size is concerned with the student-to-teacher ratio during instruction and the corresponding levels of explicit instruction that can occur. Increased group size (above four students per teacher) can lead to diminished student outcomes due to fewer opportunities to receive corrective feedback (e.g., Torgesen et al., 2007).

    The instructional period or length of each session can range from a few minutes to double-period blocks in secondary schools. In general, student attention and cognitive load may wane under prolonged periods of intervention exposure, whereas periods of instruction that are too brief may not provide time for sufficient strategy acquisition or independent practice.

    The third factor, frequency, refers to the number of times students are instructed per week. Akin to the challenges of instructional period, instruction that occurs too often or not enough will impact outcomes.

    Duration, the final variable, is concerned with two factors: (a) What is the optimal total number of sessions students should be instructed? and (b) What is the optimal length of time from start to finish? Each of these factors can support and/or inform decisions about the others, and a failure to take any of these factors into account may limit the efficacy of or adversely affect an intervention.

    In short, in order for RTI efforts to be effective in improving student performance, we must make certain that interventions are not only taught correctly but that they are being taught with sufficient intensity.  While both fidelity and dosage are relatively easy constructs to understand, they are much more difficult to implement effectively in practice.  The potential complexity of ensuring that any RTI effort meet high fidelity and dosage standards highlights one of the challenges to effective implementation of RTI programs — however, if these two issues are successfully addressed, there will be great payoff in terms of student outcomes.

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