Treatment Dosage and Duration
Two important delivery characteristics of an intervention are the duration or the time length of a treatment (e.g., 6 months, 12 months) and treatment dose, which refers to the amount of treatment. Dose is typically measured in terms of the number of sessions, contact time (e.g., minutes), number or frequency of contacts, or some combination (e.g., 12 weekly 60-minute sessions). Dose parameters may be applied to an interaction between a participant and a member of a research team (e.g., counseling session with a therapist), or some aspect of an individual’s behavior (e.g., number of fruits or vegetables eaten, or minutes of exercise), or across several components of an intervention. In technology-based interventions, a dose parameter may reflect the number of uses of the system or an aspect of a particular software program. For example, in the PRISM trial, real-time data were collected on participants’ use of the overall PRISM system as well as the use of each individual feature each day and over the 12-month duration of a trial.
A fundamental question in the design of the delivery characteristics of a behavioral intervention is: How much exposure, and to what, is enough? Clearly, this issue has a significant impact on treatment outcomes as well as the cost of a study, participant burden, and feasibility issues surrounding future implementation of an intervention at later stages of the pipeline. Insufficient doses of a treatment may contribute to Type II errors (the failure to reject a false null hypothesis; failing to detect the impact of a treatment that is present). On the other hand, large amounts of a treatment may negatively impact on the cost and feasibility of a trial and be burdensome to participants. This may in turn have a negative impact on participant retention, which can threaten the internal validity of a study. In addition, it may limit replication of the intervention and broad-scale implementation.
The goal, of course, is to find the optimal balance between demonstrating the effectiveness of a treatment (if there is one) and cost and feasibility issues. There is no one recommendation for the optimal dose. It depends on the nature of the intervention, the target population, and the context/setting. For example, the REACH II intervention trial involved 12 sessions (nine in-home face-to-face and three via technology) delivered over 6 months. A translation of the intervention was subsequently used in four Area Agencies on Aging (AAA). This version of the intervention was trimmed to enhance the feasibility of implementing the intervention in social service agencies and included seven sessions (four in-home face-to-face and three via telephone) delivered over 4 months. The results indicated that the translated version of the intervention appeared to be effective in terms of achieving a positive impact on caregiver outcomes such as burden, depression, and social support (Burgio et al., 2009).
Overall, determining the optimal dose of an intervention can be guided by a review of the relevant literature and through evaluation of the impact of the intervention in varying doses. For example, sequential, multiple assignment randomized trials (SMARTs) (Lei, Nahum-Shani, Lynch, Oslin, & Murphy, 2012) may be used to evaluate the impact of an intervention at varying doses. These types of designs allow for tailoring of the intervention components in the same trial.
Once the duration and dose of an intervention have been established, it is critically important to develop a system to track the planned and unplanned contact with study participants in all conditions during the course of a trial. This will allow the measurement of the dose delivered and the assessment of dose-response relationships. It is also important to determine how much dosage (e.g., number of sessions or time spent) of an intervention constitutes sufficient exposure and/ or “completion” of the intervention protocol to evince a benefit. Additional factors to consider are issues related to the delivery schedule (e.g., once per week vs. once per month), flexibility in scheduling, and whether booster sessions are appropriate.