Comparison groups in yoga research: A systematic review and critical evaluation of the literature

Highlights

• Our systematic review of control and comparison groups of yoga yielded 128 RCTs.

• 65 included only a passive control and 63 included an active comparison group.

• Physical exercise, relaxation, and education were the most common active groups.

• Use of active controls in yoga research appears to be slowly increasing over time.

• Future yoga research should carefully consider appropriate control conditions.

Summary

Objectives

Comparison groups are essential for accurate testing and interpretation of yoga intervention trials. However, selecting proper comparison groups is difficult because yoga comprises a very heterogeneous set of practices and its mechanisms of effect have not been conclusively established.

Methods

We conducted a systematic review of the control and comparison groups used in published randomized controlled trials (RCTs) of yoga.

Results

We located 128 RCTs that met our inclusion criteria; of these, 65 included only a passive control and 63 included at least one active comparison group. Primary comparison groups were physical exercise (43%), relaxation/meditation (20%), and education (16%). Studies rarely provided a strong rationale for choice of comparison. Considering year of publication, the use of active controls in yoga research appears to be slowly increasing over time.

Conclusions

Given that yoga has been established as a potentially powerful intervention, future research should use active control groups. Further, care is needed to select comparison conditions that help to isolate the specific mechanisms of yoga's effects.

Introduction

Research on yoga interventions is rapidly proliferating¹; researchers are studying the effects of yoga on a wide range of mental and physical health conditions. Researchers have demonstrated preliminary efficacy for many conditions, including arthritis² stress³ metabolic syndrome⁴ asthma⁵, pain⁶ and depression.⁷ However, the literature is replete with many contradictory findings and generally characterized as inconclusive due to the weak design of many studies.4, 5

A common methodological limitation of many yoga intervention studies is the lack of an adequate control condition. Although trials of the effects of yoga have often been conducted without a control condition, using pre-post designs, more recent research typically employs a control condition as well as randomization and other elements of experimentation.¹ Selection of an appropriate control group is very important in intervention studies because comparison of the intervention and control groups allows researchers to isolate and test the purported active ingredient of the intervention and hold all other factors constant.⁸

Unlike in studies of drug effects, where a placebo is relatively straightforward, selection of a control condition is much more complex for behavioral interventions.⁸ Yet because of its centrality to the interpretation of the study results, the comparison group is essential and must be selected with care and deliberation. The appropriateness of a comparison group depends on the specific research question being asked, so that researchers can be confident that differences found between groups is due to the hypothesized active ingredient of the yoga intervention.9, 10 Thus, the theoretical mechanism through which the yoga is presumed to operate and the comparison condition would ideally include all of the elements of the yoga condition minus that active ingredient. Of course, in behavioral interventions, the “active ingredient” is complex and difficult to identify. Still, when interpreting the results of a study, it is critically important to have an adequate condition to which to compare the yoga intervention group so that the effective ingredient(s) of the yoga can be isolated and tested. Yet, perhaps because of the heterogeneous nature of yoga,11, 12 the rationale for selecting the comparison is rarely explicated in published reports.¹⁰

One common solution is a passive control, such as a wait-list or usual treatment.⁹ Wait-list typically means that participants in the comparison group get usual care for the intervention period but then are able to receive the intervention after the study ends, while participants in a usual care condition simply receive no additional treatment. Participants in wait-list or usual care groups are sometimes asked to avoid changing their usual treatment unless medically necessary. While inclusion of these groups in the research design controls for the passage of time and natural course of a given problem or condition, they are limited as controls in mind-body interventions such as yoga, given the many nonspecific factors that may confound results or even constitute important elements of the intervention.¹⁰ However, passive control groups are reasonable in early stages of research, given the practical demands of active comparison groups, whose inclusion require investigators to recruit many more participants and to deliver the comparison condition.

However, as research advances, the need for more active comparisons becomes obvious. Yoga involves ongoing interaction with an instructor, which can have substantial effects on some outcomes. In addition, a passive control does not account for other potential nonspecific factors, including expectancy effects, attention and time spent in the intervention.10, 11 Further, passive control conditions do not typically entail the same amount of participant burden or investment. In addition, some researchers have argued that using a waitlist control may spuriously amplify the difference in treatment effect between the intervention and the control because people assigned to the waitlist may expect to not get better without active treatment.8, 13

Study designs that include information-only or self-study control groups in which participants are given informational materials to read on their own are somewhat closer to designs that include an active comparison. Although a few especially motivated participants may read everything they are given and take action that will change their health, these control groups are usually expected to have minimal effects across the whole group of participants randomized to that condition.¹⁴ However, this option may have some appeal to potential participants during recruitment and alleviate the potential disappointment some participants experience when getting randomized to the “usual care” condition.¹⁰

Investigators may also employ active control conditions such as relaxation or exercise in their studies.¹⁰ These control conditions may constitute a stronger comparison group for discerning the effects of yoga, but because few authors reporting on yoga interventions explicitly state the mechanism through which they believe the yoga will affect the outcome, the rationale for the choice of control is often not provided. Active control conditions may or may not control for multiple aspects of the intervention (time, attention from a provider, group setting). Active control groups provide an important practical advantage in research as well, making participation in studies more appealing and improving retention rates.⁸

To better understand the current state of yoga intervention research, we undertook a systematic review of the control groups employed by investigators in randomized controlled trials (RCTs) of yoga, which are more methodologically rigorous and may be more heavily weighted in determining evidence regarding yoga's effects. For those studies in which an active control was included, we attempted to examine the rationales provided for that choice. With this information, we endeavor to provide some direction for future studies of yoga interventions.