We searched the literature from 1997 to 2013 related to workplace exercise and the control of occupational pain and disability of the shoulder and neck/shoulder. The literature search was conducted through PubMED in addition to databases for CINAHL, CISILO, Embase, Ergonomics Abstracts, Health and Safety Science Abstracts, HSELine, OSHLine, PubMed, SportDiscus, Web of Science, and WorldCat using the search terms shown in Table 1. Except where noted, the PubMED search used these terms as a controlled MeSH vocabulary. The other databases were searched on text words. The emphasis of the review was on studies with occupational study populations in which the exercise program was administered as an intervention to address shoulder pain, symptoms, or disability. The review included studies for which exercise was primarily a workplace intervention; however, several studies included occupational groups and addressed occupational shoulder or neck/shoulder pain, but the program consisted of home exercises (or home in combination with the workplace) or a program at a therapy clinic. These studies were included. The strategy was to exclude studies in the areas of athletic and sports performance, general fitness training, and the strength and conditioning literature, which were general nonoccupational study populations and not considered workplace-administered exercise. The review was defined more broadly to include articles in which the shoulder or neck/shoulder was revealed in the search strategy because a number of exercise intervention studies include neck/shoulder pain outcomes together. Studies were excluded if a publication was not available in English or if the study was not prospective in design.

The initial search returned 393 references, which were manually filtered for possible relevance. This resulted in 116 primary references identified for which abstracts were reviewed by a single author. Studies were frequently rejected because the workplace intervention did not include exercise. After review of these abstracts, full articles were retrieved for 46 studies. An additional nine studies and abstracts were identified from reference lists. After further screening of full articles and combining duplicate publications, 38 relevant studies3,4,6,8,10,21,25–56 were included in the final review.

Data extraction from the articles was performed by a single author and entered into a form indicating the publication year, country, occupational group from which participants were recruited, inclusion criteria (diagnosis), exercise modality, exercise environment, study design, measure of program compliance, progressive nature of exercise training, program duration, sample size, percentage continuation with program, exercise training sessions frequency, exercise session duration, total exercise time, outcomes, and statistical summary. We did not document the funding sources because we did not think commercial interests were a potential source of bias in these studies.

Each study was assessed according to the levels of evidence, considering its design/randomization, consistent with evidence-based medicine guidelines.57 Level I evidence (strongest) was assigned to RCTs, and Level II to lower quality control group trials with group-level randomization or matched controls. Level IV (weakest) was assigned to case series designs and study designs lacking a nontreatment control group (eg, comparing two types of exercise programs). The review adhered to guidelines of Preferred Reporting Items for Systematic Reviews and Meta-analyses (PRISMA)58 with some key exceptions. A protocol was not registered for this review. Second, risks of bias in individual studies and across studies were not formally assessed beyond the randomization of treatment, as described previously. The most notable source of potential bias, prevented by blinding of participants to the intervention treatment, is infeasible with an exercise program. We did not document concealment of allocation of studies because this was rarely reported in the articles. Third, no quantitative synthesis of results, or meta-analysis, is reported.

Studies were classified according to a positive or null effect, indicated by a statistically significant (0.05 alpha error criterion) reduction in musculoskeletal outcomes attributable to exercise, without the confounding presence of co-intervention. If no statistically significant improvement in outcome was associated with exercise, the study was classified as a null effect (no difference between exercise group and the nonexercise control group).

The positive studies tended to more often involve SRT as an exercise modality with specific neck/shoulder disorders or nonspecific neck/shoulder pain as inclusion criteria. SRT was a component in 10 of the 20 positive studies and all five positive studies that included participants with specific neck/shoulder disorders. Only one null study8 included SRT (in combination with stretching). That was also the study of longest duration (104 weeks), was a home-based exercise program, and reported low compliance.

Several studies reported a null effect of exercise specific to shoulder outcomes. Tsai et al.51 concluded that 12 weeks of general physical fitness exercise, GPE (emphasizing aerobic exercise), was effective based on reductions in neck, wrist, and back pain to office workers and laborers with undescribed symptoms. However, a statistically significant reduction in symptoms was not found for shoulder pain. Similarly, the RCT cross-over design by Sjogren et al.48 reported a significant reduction in neck symptoms, but not shoulder, after 15 weeks of light RT consisting of six dynamic movements at 30% of one repetition maximum. In an eight-week cohort study of the effect of RT on musculoskeletal disorders of 19 shellfish gatherers, Rodriguez-Romero et al.46 reported reductions in back pain, but no change in pain for the neck, shoulder, or upper or lower extremities. In a 32-week study of 50 office workers, Macedo et al.40 reported no difference between exercisers and nonparticipating controls on reports of neck pain but reported positive benefit in back and wrist pain. The exercises appeared to be highly unstructured, described as emphasizing employee enjoyment, including stretching and “playful and recreational activities” on an individual basis, as well as in pairs and in groups. Compliance with this program was not reported.

Borstad et al.8 followed 240 asymptomatic construction apprentices, assigned (by group) to either an exercise or control group, over two years. Onset of new shoulder pain (14.4% overall) was not statistically different between the groups, indicating that the SRT/S exercise program did not have a preventive effect. Compliance with the program was reportedly low, perhaps attributable to the long study period.

Hagberg et al.35 reported significant increases in shoulder strength, increases in arm motion performance, and decreases in rating of perceived exertion (RPE) over a 4–24-week RT rehabilitative exercise program for nonspecific neck/shoulder pain in female industrial workers. The trial lacked a nonexercise control group and compared two types of exercise: a strength-based (5 second maximal contractions) and an endurance-based (two-minute contractions at 25% of maximum) exercise program. No differences in pain rating between the two treatment groups were found, and differences in reported pain from baseline (assessed at four-week intervals) were inconsistent. This study was classified at Level IV and a null effect.

Other studies reported no effect of exercise relative to a nonexercising control group. Van den Heuvel et al.53 randomly assigned exercise described as “four physical exercises lasting 45 seconds each” every 35 minutes to 268 office workers using computers. The specific exercises were not described, but appear to have included stretches. The three minutes of exercise was part of a five-minute work break (every 35 minutes) given to office workers with upper limb musculoskeletal complaints. After eight weeks, no differences were found relative to a group who simply took a five-minute rest break prompted by software interruption. There were also no significant differences between groups in pre- post-intervention complaints of, or missed time due to, neck or upper limb disorders. A crossover design study of a video-based six-week Qigong training program (classified as MA) concluded that the exercise slightly reduced neck disability among 42 office workers.49 However, no change in neck/shoulder pain intensity was found. Compliance with this program was reportedly high, with 83% of the group sessions attended. This may, in part, reflect a reduction in work time to accommodate participation in the program, which was prior to employees’ lunchtime.

Schwarz et al.47 examined the effect of 2.5 hours of weekly GPE compared to equivalent reduced working hours (37.5-hour work week) and no reduced work hours over one year among a homogenous sample of healthcare professionals. The reduced work hours group received no reduction in monetary compensation. The specific exercise was at the choice of the individual so long as it met the criteria of vigorous activity of 55%–89% max HR (consistent with GPE classification). The group × time interaction effect on a composite musculoskeletal symptom score did not reach statistical significance (P = 0.06) but a trend toward a benefit of the exercise program was evident. The authors discussed the fact that reduced work hours were not offset by proportionate reduction in expected productivity and that this might have increased work demands. All groups (including controls) reported regular levels of exercise at baseline, which may have contributed to ceiling effects with regard to health improvement. All groups (even the controls) increased physical exercise over the study period, suggesting a possible contamination bias.

Three additional studies of RT were classified as having a null effect on nonspecific neck/shoulder pain. Lundblad et al.39 evaluated a Feldenkrais technique program (classified as MA) and a group-based physiotherapeutic intervention (classified as RT) that included group-based exercise and exercises to be practiced at home by female employees working with nonspecific neck shoulder symptoms. These programs were compared to a nonexercise control group over a 16-week period. Specific complaint indexes in the neck and shoulders, ratings of pain, and work disability did not change differentially over time between exercise and control groups. The 40% dropout rate in this study appears to be high. Compliance with the exercise among the non-dropouts was not reported, beyond meeting a minimum of 50% exercise participation. Maher et al.41 evaluated a 26-week rehabilitative RT program using elastic band resistance on 11 manufacturing assembly workers with nonspecific neck shoulder pain. The sample size was too small for parametric statistical testing, and the single statistically significant finding (nonparametric) was reported in the form of a response to an individual disability question from a 16-question index. Viljanen et al.55 reported no effect of a one-year, three-times-weekly, 30-minute dynamic dumbbell strength training (RT) intervention or a relaxation training intervention relative to a control group on reductions in neck pain in symptomatic female office workers. However, notable about this study is that it was the lowest reported compliance of the studies reviewed. After 12 weeks, participants had completed 39% of strength training sessions; after one year, this had fallen to 34%. Compliance with the relaxation training fell more precipitously – from 42% at 12 weeks to 22% at one year.

Five studies were classified as inconclusive, three of them because ergonomic co-interventions were part of the experimental treatment and were believed to confound the effect of exercise.33,38,44 Povlsen44 evaluated a 12-week rehabilitation program that included physiotherapeutic exercise to improve flexibility and strength that emphasized the hand, wrist, and forearm. The study lacked a control group (Level IV), and the program included ergonomic workstation modifications. Rating of pain decreased both before and after bouts of typing as a result of the overall program. Because of the ergonomic modifications (co-interventions), it is not clear whether the effects can be attributed to exercise. LeClerc et al.38 included an exercise program component in a matched control group trial (Level II) including hospital, office, and warehouse workers. Shoulder morbidity score (based on pain intensity/duration and disability) decreased significantly in the intervention group relative to controls. The exercise, described only as “physical training in an exercise room” appeared to be the emphasis of the intervention, but the intervention also included ergonomic program components. Feuerstein et al.33 compared an ergonomic intervention with a combined ergonomic and stress reduction group. This study incorporated exercise insofar as active muscle relaxation and diaphragmatic breathing were part of the stress reduction intervention. Both treatment groups exhibited significant reductions in symptoms (DASH) and functional limitations. This study was classified as inconclusive with regard to the breathing exercise because no differences could be detected between the ergonomic intervention group and the combined treatment group. The interventions reduced pain but could not be attributed to the deep breathing relaxation response exercises. Because the ergonomic intervention alone reduced pain equivalently suggests that the relaxation exercises may not have been effective.

Bernaards et al.30 evaluated “work style” and “lifestyle physical activity” interventions over a one-year period on the reduction of neck and upper limb symptoms among 466 computer users in the workplace. The lifestyle physical activity intervention counseled participants to incorporate moderate to heavy intensity physical activity into the lifestyle. The authors found reductions in pain attributable to the work style intervention but none that could be attributed to the physical activity (counseling) intervention. However, the physical activity counseling did not actually increase (self-reported) physical activity. Lacking a measurable increase in physical activity as a result of the intervention, the effect of the exercise intervention cannot be determined.

An RCT with an alternative treatment control reported no difference between 10 weeks of Mensendieck/Cesar exercises (postural/MA) and a strength and fitness exercise program standard to physiotherapy.54 No statistically significant differences were reported between exercise therapy approaches, and results for within-group time effects were not reported. A benefit of exercise relative to no exercise cannot be discerned.

Drawing conclusions about the exercise modalities and their efficacy for the control of neck/shoulder pain is difficult because of the nonstandardized outcomes in the studies (symptoms, pain, disability, or lost work time), the nature of the neck/shoulder symptoms of the study population, and the combination of exercise modalities in several studies. The wide range of exercise program designs across these studies is a reflection of the multifaceted benefits of exercise to overall health and the nonspecificity of musculoskeletal pain (beyond specific neck/shoulder pain) that these programs were, in many cases, addressing. Though our review focused on the neck/shoulder outcomes reported, many studies included programs designed to address broader musculoskeletal health.

Several program designs that targeted specific neck/shoulder symptoms include components from more than one exercise modality, most commonly SRT and stretching.3,8,10,32 An attribution of one specific exercise modality to the positive effect in such a study is not possible, and it may not be useful to draw conclusions about the efficacies of various exercise modalities in isolation when the most effective program combines modalities. Nonetheless, most studies include exercise modalities consistent with our framework, and general trends across the exercise modalities can be observed. The most compelling evidence, by way of studies classified with a positive exercise effect, supports SRT as a therapeutic intervention for specific shoulder neck/shoulder disorders (ie, trapezius myalgia, subacromial impingement syndrome, and rotator cuff tendinitis) as well as nonspecific neck/shoulder pain and symptoms. The evidence in support of the use of general RT is mixed, with four Level I positive studies, one Level II and one Level IV positive study, but seven null studies (three with Level I evidence). Among the six positive Level I/II studies including stretching components, four of the programs combined stretching with SRT or RT.10,32,43,56 Another study reported a positive effect only on a composite musculoskeletal symptoms index that was nonspecific to any musculoskeletal region.42 Another did not report inclusion criteria symptoms/diagnosis, and the outcome was a measure of soreness prevalence and not intensity.52 An additional null study53 did not describe exercises sufficiently to be classified definitively, but clearly included stretching exercise, and may have been exclusively stretching-based. No positive Level I studies were found in which stretching was the sole exercise modality. We interpret this evidence as indicating that general physical exercise (defined by activities higher in cardiorespiratory demand, ie, “aerobic exercise”) and stretching, in the absence of RT, are less effective than SRT as workplace-based exercise to reduce neck/shoulder symptoms or pain. There were no Level I or II positive studies on neck/shoulder pain reduction including what we collectively grouped as movement awareness modalities (eg, deep breathing/relaxation, Tai Chi, and Qigong).33,49,50,54 These studies were generally downgraded in evidence because they either lacked a control group or combined the exercise with other interventions. There were two Level I null studies of these modalities. Higher quality positive studies of these modalities are needed to more adequately demonstrate their efficacy.

The multiple varied exercise modalities make it difficult to interpret study findings with regard to how much workplace exercise is beneficial, ie, a minimum effective exercise dose – considering frequency, duration, and intensity. We were able to reconstruct the exercise program time in the majority of studies. In the aggregate, there does not appear to be a relationship between longer weekly exercise time and positive effects. It could be concluded that the opposite trend was evident and that little benefit is evident beyond 90 minutes/week of exercise. Studies with 90 min or more of weekly exercise were more likely to be classified as null (73% of null studies for which exercise time could be reconstructed) than as positive (only 31% of positive studies involved more than 90 minutes of weekly exercise). One-half of the positive studies involved weekly exercise of 60 minutes or less. Median weekly exercise time over positive studies was 70 minutes and for null studies was 100 minutes. Minimum effective exercise time is important from a physiologic standpoint and, in a workplace exercise program, from a cost/benefit standpoint to the employer if the exercise program takes place on paid time. Our findings differ somewhat from the conclusion of a recent review of interventions for upper extremities (UE) CTDs of computer users.65 These authors created symptomatic and asymptomatic best practice models using occupational therapy principles. For symptomatic computer users the authors suggest “…stretching, range of motion, strengthening and posture exercise involving shoulders, forearms, wrists and digits of the hand” (p. 167) with a specific exercise dose of three weekly sessions of one hour – primarily based on Omer et al.43 In our review, that study43 represented the second highest weekly exercise dose (in terms of exercise time) among the studies we classified as positive. Importantly, our review, unlike that of Goodman et al.65, does not address distal upper limb conditions, was not limited to occupational computer users, and had the benefit of more recent studies. Nonetheless, our review does not justify three hours of weekly exercise as a recommendation for the shoulder.

The distribution of exercise session frequency and duration has been addressed in few studies other than Andersen et al.28 who concluded that in distributing one hour of exercise time, three weekly sessions of 20 minutes appears to be preferable in reducing neck/shoulder pain relative to a single, weekly 60-minute session or nine 7-minute session per week. The authors note that compliance (adherence) was lower for the single, longer weekly exercise session and that training load progression (gains in shoulder strength) were inferior in the 7-minute sessions. However, the definition of “regular adherence,” defined by participation in at least 20 minutes/week of exercise (only one-third of the prescribed program dose), combined with the differential adherence across the three programs obscures an assessment of the actual positive dose–response.

Silverstein et al.66 conducted a one-year prospective study of workplace exercise on musculoskeletal symptoms and reported no effect of the exercise program on neck and upper limb symptoms. The authors suggested that one year might be insufficiently short to assess an effect on discomfort reduction among employees working with discomfort for years. If that observation was a call for longer duration studies, it has not been answered. Our review of 38 workplace exercise studies (relevant to the shoulder or neck/shoulder) dating back to 1997 (nearly 10 years after Silverstein et al.66) includes only two studies even reporting a duration longer than 52 weeks.8,36 These studies were both classified with null effects. Further, the cumulative exercise time in Silverstein et al.66 (14 minutes daily exercise for 52 weeks) exceeds that of all but three of the studies for which we could estimate this – all three of which were classified as null.8,47,55 The challenges in conducting workplace studies of longer duration are clear, and the lack of any reported, let alone positive, studies that have followed workplace exercise beyond one year makes it difficult to draw conclusions about longer term effectiveness on neck/shoulder pain. The present review reveals clear differences between studies of shorter versus longer duration. Of the 20 studies we classified as positive, only 2 were trials longer than 20 weeks duration, while over half (7 of 13) of the classified null studies exceeded 20 weeks. This is consistent with a differentiation of shorter term and longer term outcomes and an observed absence of evidence for longer term effectiveness.59 Studies of longer duration are more difficult to conduct and perhaps result in less publication bias, such that null studies of longer duration are more likely to be published. This is speculative, but consistent with a recognized publication bias against null studies with small sample size59 (also less difficult to conduct).

Variation in studies with regard to program compliance, when this was even reported, creates challenges in interpretation of exercise program effectiveness.67 The studies are not standardized in reporting the handling of noncompleting dropouts (intention to treat), or in describing actual compliance with the prescribed exercise dose for participants completing baseline and follow-up assessment study phases. The ability to interpret exercise program compliance is also critical to conclusions regarding training dose. Compliance was self-reported in 40% of studies for which we could extract this information, and some studies set arbitrarily low thresholds of acceptable/unacceptable compliance without assessing the effect of low compliance on exercise dose. For instance, Andersen et al.28 reported a positive effect of a 60 minutes/week (prescribed) exercise program, but 39% of participants reportedly attended only one-third of that exercise dose (a single 20-minute session). Andersen et al.27 reported that adherence to the training doses was no different (66%) between a 2-minute and a 12-minute daily exercise bout, in spite of the sixfold difference between exercise session times. If the compliance percentage is accounted for, the actual exercise dose averaged only 78 seconds/day in the 2-minute daily exercise group.

To the extent in which compliance was reported in the reviewed studies, an obvious trend did not emerge with respect to exercise modality and compliance. However, among the 20 studies for which we could reconstruct compliance, an inverse trend was apparent between study duration and reported compliance. Over 40% of the variation in compliance (percentage of exercise completed) may be explainable by program duration (in weeks). Other authors have noted key differences between short- and long-term exercise compliance67 as well as the complexity of the multiple factors that affect exercise compliance68,69 and its measurement validity.70 Our definition of workplace exercise, to be broadly inclusive of eight studies of home exercise administered with workplace oversight, allows a comparison of the home exercise environment to the workplace exercise environment. Compliance appeared to be notably higher when exercise was performed at the workplace as opposed to the home environment. This suggests that the workplace may have advantages to the promotion of exercise for work-related neck/shoulder pain.

Other than identifying study populations by occupational group, these studies reveal little about the differences in work environments and work cultures across various occupations. Studies have included a variety of occupations, but the study populations in half of the articles reviewed were computer/office workers. This may be representative of the composition of the modern workplace; or it may also reflect the relative convenience of enrolling and following a larger number of workers at a single study site, with homogenous workplace exposures and work organization characteristics, which allows exercise breaks to be scheduled into the workday. Workplace physical exposures for computer users in office work, as well as their effect on neck/shoulder pain, are quite different from those of the construction worker, assembly line worker, and other occupations.

The eight Danish studies deserve mention for the positive effects they report in regard to resistance training (RT and SRT) exercise modalities. Of the Danish studies, seven were positive and one inconclusive. All studies from Denmark included RT/SRT (six studies) or RT (two) studies. Thus, the high percentage of positive findings that evaluated SRT may be somewhat confounded with their being conducted in Denmark (62% of studies classified as positive Level I/II evidence with SRT exercise modality were Danish studies). The positive findings in these studies may relate to superior aspects of the exercise program or the study designs. Alternatively, this trend may reflect other sociocultural factors in the Danish workplace that may not be completely generalizable across cultures. Ideally, the evidence in support of workplace exercise of the SRT modality would be more evenly represented across countries of origin.

Psychosocial and cultural factors influence acceptance of workplace exercise and, accordingly, have played a role in exercise program design and adoption. As a US example, Lee et al.71 reviewed exercises from 14 documented exercise programs for computer users. The criteria they adopted for exercise suitability considered exercises that could be performed at the employee’s workstation, most while still sitting in their chair. The authors presumed that less conspicuous exercises would result in less embarrassment and compliance would thus be improved. Consistent with this interpretation outside the US, van den Heuvel et al.53 (Netherlands) reported that 25% of exercise study participants reported an expectation of embarrassment from exercising in the presence of coworkers. These psychosocial aspects of the work environment create significant constraints to exercise program design and challenging barriers to adoption of workplace exercise. This may be particularly problematic if the more effective exercises (eg, SRT) are also the more conspicuous. It is also possible that, as workplace health/exercise promotion has become more common since the early 1990s,71 changing social and cultural norms may lessen potential embarrassment.

The importance of proximity and accessibility to exercise equipment may be more relevant to RT modalities, and its effect on compliance is evident in these studies. In the study by Ylinen et al.56, a home exercise program was designed because employees were reportedly unwilling to visit health clubs. The relatively low compliance with that study, and of home programs overall, suggests advantages of exercise in the workplace. When employees could exercise in the workplace with equipment (eg, dumb bells, resistance tubing, or bands for SRT) in designated areas in close proximity to, yet physically separated from, their workstation, Zebis et al.25 reported “regular” compliance with exercise for 85% of participants. Others have emphasized that accessibility to exercise classes was important to participation and that participants indicated that a significant reason for participation was that the exercise was easily accessible at the workplace.50 Workplace programs may be a compromise in the trade-off between exercise programs in a health club setting, requiring more travel to and from specialized equipment, and the convenience of home-based programs that appear to exhibit inferior program compliance.

In the studies reviewed, we found little information describing administrative aspects of the exercise programs that affected participation incentive. In home programs, it was assumed that the time burden was borne by the employee, and, not unexpectedly, compliance was substantially lower with home exercise. When exercises were performed in the workplace, the expectation of productivity compromise and/or paid versus unpaid work hours was often unclear. In several studies, the exercises were reported as being conducted during or before lunchtime breaks43,49,50 or at the beginning or end of the shift,34,51 but it was not necessarily clear if this was during compensated time. In other cases, it was clear; for example, when one-third of participants report a negative expectation of a loss in productivity due to the scheduled exercise breaks as part of that program.53 In many cases, employees were clearly given flexibility to exercise during normal work hours, or were prompted by scheduled work breaks, and were compensated during that time.3,21,25,29,36,42,48,53 In these studies, voluntary program participation is assumed. A notable exception, and interesting observation on program compliance, is a study of “mandatory” exercise (GPE meeting criteria of 55%–89% maximum heart rate for 2.5 hours/week).47 These authors reported over 100% compliance (as self-reported), by virtue of exercise time exceeding the targeted level. This study (classified as a null finding in our review) concluded: “When it was mandatory to spend the reduced work hours on physical activity, the increase in physical exercise was even greater … it follows that interventions involving a modest reduction in work hours seem to be more effective when the time is spent on physical exercise” (p. 187). The concept of a 2.5-hour (~6%) reduction in workweek hours so that workers can exercise may be unrealistic in the US without demonstrating to employers that the 6% can be made up elsewhere. A consideration rarely described in these studies is the added job demands and musculoskeletal stress when administratively reduced work time for exercise is not accompanied by a proportionate reduction in expected work output.47,66

The findings of this review suggest areas of future research related to workplace exercise efficacy for control of neck/shoulder disorders, and how future research could be improved. We suggest that future research address (1) exercise modality – studies that adequately describe the type of exercise activity and exercise stimulus; (2) compliance – studies that evaluate the degree to which compliance/adherence affect workplace exercise program efficacy. Mandatory exercise participation in research may not be feasible, and introduces ethical considerations. However, participation incentives should be described and the effects of incentives on workplace exercise participation and compliance could be studied; (3) exercise program duration – longer term studies are still needed to address the extent to which short-term benefits (reduction in pain/symptoms) persist over the longer term when workplace physical demands are constant; and (4) exercise time/dose – studies are needed to determine minimum effective exercise dose – the least amount of exercise time resulting in positive effects on pain/symptom outcomes. Finally, it is suggested that outcomes and instruments for measuring neck/shoulder pain and symptoms be standardized to improve comparability across studies and better facilitate meta-analysis.