The Effect of Financial Compensation on Health Outcomes following Musculoskeletal Injury: Systematic Review

The effect of financial compensation on health outcomes following musculoskeletal injury requires further exploration because results to date are varied and controversial. This systematic review identifies compensation related factors associated with poorer health outcomes following musculoskeletal injury. Searches were conducted using electronic medical journal databases (Medline, CINAHL, Embase, Informit, Web of Science) for prospective studies published up to October 2012. Selection criteria included: prognostic factors associated with validated health outcomes; six or more months follow up; and multivariate statistical analysis. Studies solely measuring return to work outcomes were excluded. Twenty nine articles were synthesised and then assessed using GRADE (Grading of Recommendations Assessment, Development and Evaluation) methodology to determine evidence levels. The results were mixed. There was strong evidence of an association between compensation status and poorer psychological function; and legal representation and poorer physical function. There was moderate evidence of an association between compensation status and poorer physical function; and legal representation and poorer psychological function. There was limited evidence of an association between compensation status and increased pain. In seven studies the association depended on the outcome measured. No studies reported an association between compensation related factors and improved health outcomes. Further research is needed to find plausible reasons why compensation related factors are associated with poorer health following musculoskeletal injury.


Introduction
Injury is a leading cause of disability worldwide and musculoskeletal injuries commonly occur within compensation systems for road traffic crashes and work place incidents [1,2]. In previous studies associations have been found between: legal representation and poor general health, and greater disability [3,4]; litigation and psychological distress [5]; legislative change and increased pain [6,7]; and claim lodgement and poor general health [8,9].
Identifying predictors of poor health outcomes following injury provides valuable information for risk assessments, targeted interventions, policy initiatives and future research to improve recovery. Furthermore, determining whether compensation related factors are associated with specific health outcomes particularly those including the constructs of pain, disability, physical and mental health is important given the prevalence of injury, societal concern with ongoing disability, and associated costs. Therefore, we considered a comprehensive literature review was required to determine whether the association between compensation related factors and poorer health outcomes is reported across a wide range of musculoskeletal injuries, prognostic factors and health related outcomes.
Compensation systems operate in a highly contextual environment. Policy relevant research that provides information to assist scheme administrators, regulators and researchers to promote injury recovery and improve scheme efficiency has merit, particularly if the association between a compensation related factor and health outcome is shown to be modifiable [10].
In previous studies, compensation tends to be classified as a single variable, rather than exploring separate elements of compensation such as scheme design, claim duration or legal representation. Further, compensation is not usually the primary focus of studies investigating injury recovery [11][12][13][14]. To the authors' knowledge five reviews have focused on the association of compensation with poorer health following injury [15][16][17][18]. These reviews have disparate injury groups such as road trauma, post-surgery, traumatic brain injury, and whiplash. Health outcomes are also clustered under the umbrellas of mental health, satisfaction, general health and disability. Most of these reviews conclude that compensation related factors are associated with poorer health [15][16][17], whilst one review cited reverse causality bias as a methodological issue (i. e. does exposure to compensation lead people to poorer health or does poorer health lead people to claim compensation) [18]. Another meta-review outlined additional flaws including: poor quality primary studies; use of proxy health outcomes; and the heterogeneous nature of compensation related factors [19]. None evaluated the evidence by categorising compensation related factors and outcomes. Therefore, based on these reviews it is difficult to determine which compensation related factors are potentially associated with particular outcomes following injury.
Accordingly, the aims of this review are to identify associations between compensation related factors and health outcomes following musculoskeletal injury from prognostic and/or intervention studies. In this context, compensation related factors are those associated with compensable personal injury insurance schemes, including between or within scheme comparisons such as claim type or fault versus no fault.

Methods
We conducted a systematic review of prospective studies that investigated predictors of health outcomes following musculoskeletal injury in subjects exposed to a compensation related factor with an unexposed comparison group. The study aims and selection criteria were developed a priori.
The review included studies published in any language. The selection criteria were: • at least 18 years of age (for majority of participants); • study aimed to determine prognostic factors associated with an outcome, or to assess the effect of an intervention with compensation related factors included as covariates; • measurement of one or more compensation related factors associated with an outcome; • at least one validated health related outcome measure was reported; and • inclusion of a predictive model with multivariate statistical analysis.
Exclusion criteria were: • participants with dementia or significant pre-existing cognitive impairment; • participants with a moderate or severe traumatic brain injury, spinal cord injury, psychological or other organ and body system injuries; • studies involving only children; and • studies where the only outcome assessed is return to work with no other validated health related outcome.
Due to the diverse injury definitions, three approaches were used: definition and context (mechanism or insidious onset); diagnosis; and/or duration (acute or chronic). Only prospective studies were included to reduce the risk of bias [20]. A follow up period of six months was given to allow for injury recovery. Return to work was excluded because there is no standardised measure although it is recognised that return to work is correlated with health status.

Search strategy
Searches were conducted using Medline, CINAHL, Embase, Informit and Web of Science for studies published up to October 2012. Complete search strategies are available in S1 Appendix. The strategy was based on recommended guidelines to maximise search sensitivity [21]. Key elements involved exploding terms related to cohort studies, compensation and musculoskeletal injury. MeSH headings and text words were used in conjunction with Boolean operators and wildcards. For Informit health, law and social science subjects with key words (compensation, health and outcome) were used. Web of Science and Informit provided access to grey literature. A medical librarian was consulted to assist in developing the search strategies, which were reviewed by the authors.
Articles were initially screened by two authors (DM and PC) based on title and abstract. The full text of short listed papers was retrieved. Three investigators (DM, PC and IM) conducted a two stage screening process with two authors reviewing all papers in the second stage. Articles were not excluded based on methodological quality; this was taken into account in the quality assessment.

Data extraction, quality assessment and synthesis
The characteristics of each study were tabulated to address the aims of the review [22][23][24]. Statistical information, including reported effect sizes, for all compensation related factors associated with outcome(s) was recorded. Associations were considered significant if the 95% confidence intervals of the odds, hazard or relative risk ratios did not include 1 and/or the p-value was less than 0.05. Compensation related factors were categorised as follows: • compensation (Yes/No)-having an open claim or having made a claim versus no open claim or no claim made; • lawyer involved (Yes/No)-having sought or obtained legal representation versus having none; • claim type-having an open claim or having made a claim under a specific scheme jurisdiction (Workers Compensation (WC), traffic injury (including Compulsory Third Party (CTP)), public health coverage, private health insurance, other (such as disability insurance, public liability, victims compensation); • number of sick days in prior three years; • prior claim (Yes/No); • fault (Yes/No)-making a claim under tort (fault) or no fault insurance arrangements; and • compensation at two years (Yes/No)-whether the claim was open or closed/settled at two years.
Outcomes were categorised based on measurement constructs. Similar classifications have been used in previous publications [12,13,25]. The categories were: • physical function-generic and specific measures including recovery and disability, and physical health components of health related quality of life measures; • psychological function-diagnostic based measures and mental health components of health related quality of life measures; and • pain.
Unlike intervention studies there is no agreed quality assessment methodology for systematic reviews of prognostic studies [24,[26][27][28]. However, there is some guidance on assessing study quality and risk of bias [21-23, 26, 27, 29]. Aspects such as scoring remain controversial, especially for assessing the effect size of an intervention [23,[30][31][32]. For pragmatic purposes and to provide a meaningful conclusion we followed the methodology used in similar prognostic systematic reviews where a summary score was used [11,14].
The quality assessment criteria address six areas of potential bias: study participation; study attrition; prognostic factor measurement; outcome measurement; confounding measurement; and analysis [23]. Each criterion in Table 1 specifies a bias and is assigned "Yes" or "No" with "Yes" scores being totalled (maximum score is 18). Further details are available in S2 Appendix. All papers were reviewed by two authors (DM and PC) independently. Discrepancies were resolved by consensus and/or consultation with two other authors (IC and IH). A score of 15 or over was deemed high quality, moderate quality was 12 to 14, and low was 11 or below. Although arbitrary, this division provided a fairly even distribution of scores and reflected the study quality.

Grading quality of evidence
Data analysis was based on recommendations from the GRADE (Grading of Recommendations Assessment, Development and Evaluation) working group. GRADE classifies strong, moderate and limited evidence based on: the number of papers; study design and quality; and the consistency and directness of results [28]. The levels are illustrated in Table 2. This methodology has been used in similar systematic reviews [11,12,14,33]. Inconsistent evidence refers to the negative effect of a factor in one study with a positive effect in another study regardless of study quality. For example if high quality studies showed findings in one direction and low quality studies in another; this would be considered inconsistent. In setting out this paper the authors referred to the PRISMA statement to ensure reference to all relevant reporting items [24].

Study selection
The search results and study selection process are illustrated in Fig. 1. Initially, 391 papers were independently reviewed by one investigator (DM, PC or IM). Full texts of the remaining 89 papers were independently examined by two investigators (DM, PC or IM). Reasons for exclusions are explained in S3 Appendix. In summary, they were: no predictive statistical model and/or multivariate analysis (n = 10); compensation related factor not measured as a predictor (n = 15); retrospective studies (n = 22); compensation only cohort without additional compensation related factor for comparison (n = 4); no validated health outcome (n = 6); and/or majority of cohort without musculoskeletal injuries (n = 2). Often 'prospectively collected data' were used but the study hypothesis and design were initiated post hoc after routine baseline data collection during the follow up period; these were by definition retrospective. Hand searching of reference lists and personal communication with experts minimised the potential for missing papers. Ultimately, 29 papers met the inclusion criteria.
In addition, ten papers reported results from overlapping cohorts. Only one paper from each cohort was included to avoid over representation of one population by taking into account: the range of compensation related factors and outcomes measured; injury type/s; sample size; and study quality. The studies all measured compensation status [4,8,9,[34][35][36][37][38][39][40] but the included ones measured a greater range of outcomes and/or with more applicable and comprehensive results [4,9,35,36].

Quality assessment
Following independent assessment, two authors (DM and PC) scored in agreement 91% of the time for each criterion. To resolve discrepancies: reasons for individual scores; consistent criterion interpretation; text explanations; and other referenced papers were considered. Areas of disagreement were: study participation-potential baseline measurement error and poor representative sampling (criteria S2, S3); and prognostic factor and outcome measurement-inadequate justification for each measure (criteria P2, O2). The grading of the evidence was primarily conducted by the first author (DM) with consensus review by the remaining authors (PC, IC and IH).
There were seven papers referred to other authors (IC and IH) to reach consensus. These were intervention studies, and/or had complex statistical analysis [41][42][43][44][45][46][47]. Statistical pooling was not possible due to heterogeneity of compensation related factors and outcome definitions including constructs, and follow up time periods.
Overall, 11 studies rated as high quality, 10 as moderate and eight as low. Complete scoring can be obtained from the first author.

Summary of included studies
Key study characteristics are illustrated in Table 3. Of the 29 included studies 13 were from a primary care setting or surgical clinic and 10 involved hospital recruitment. Several included both settings [44,45,48]. A further three recruited via administrative databases [43,49,50].
Injury definitions were often incomplete. Acute trauma with a hospital inception source were best described, with baseline data often collected within two weeks [3,4,9,35,44,45,48,[51][52][53]. Soft tissue injuries with an outpatient inception source were not always clearly documented [42,47,[54][55][56][57]. Furthermore, even if the inception time was stated it was not always obvious when baseline measures were conducted [46,47,58,59]. This was taken into account in the quality assessment (criteria S1, S2). However, if researchers had followed their own criteria it was difficult not to score this positively. Scores are shown in Table 4.
Sample size ranged from 65 to 3232 [43,45]. Age range was not always explicit. In 19 studies the starting age was 14-18 years, whilst in 10 studies no range was stated or it was ambiguous. There were 13 intervention studies, seven surgical and the remaining offering rehabilitation or physiotherapy services.
Follow up was a minimum of six months and a maximum of 10 years [46], the majority (15/ 29) being 12 months. Loss to follow up ranged from 0% to 52% from baseline [43,60]; this was difficult to interpret because the periods varied and/or were not reported for each outcome. Only 14 studies achieved less than 20% attrition. Most studies (n = 23) did not account for missing data but recorded loss to follow up (criterion F2). In 22 studies there was a significant difference in baseline variables between participants and those lost to follow up, or it was not explained. This was the lowest scoring criterion (F3).

Summary of compensation related factors
The studies were mostly from the United States of America (nine studies including 18 states) and Australia (nine studies from five states). There were four Canadian studies from five provinces, three Danish, two English, and one each from New Zealand and The Netherlands. The compensation schemes were predominantly WC (11/29) or a combination of WC and road traffic injury schemes (6/29). Only five studies were a road traffic injury scheme alone and one paper was for a universal accident compensation scheme. In six studies it was not stated. A description of compensation related factors and outcomes including statistics are shown in Table 4. The most common prognostic factor was compensable status (compensation Y/N) measured in 22 studies followed by legal representation (lawyer involved Y/N) measured in six. Claim type was only measured distinctly three times. The least common measures were sick leave, fault and prior claim. Compensation at two years (Y/N) is more akin to claim duration than compensable status that is: making or having made a claim, therefore it was listed separately [49].         Overall, compensation related factors were measured simply. Some specific constructs such as: fault versus no fault; eligibility; entitlements; and/or any restrictions to access entitlements were rarely mentioned. The interpretation of compensation status is potentially ambiguous and may depend on scheme design. Does it mean claim lodged or claim lodged and accepted? Furthermore, claim lodgement with or without claim acceptance and litigation (meaning legal proceedings are underway) are separate factors [36]. Finally, baseline measures of compensation related factors are likely to vary. In certain schemes legal representation can be retained at any time and/or six to 12 months is given to lodge a claim [4,9,35,44,45,49,53,61]. The timing and duration of exposure to compensation related factors was usually not documented. However, scoring for criteria (P1-3) was inclusive of compensation related and other prognostic factors. The latter were generally well justified, standardised measures with defined constructs; hence many studies (20/29) attained full scores.

Summary of health related outcome measures
Generally, studies selected more than one relevant health related outcome. Pain was the most common (14/29) usually the Visual Analogue Scale (VAS) or Numerical Rating Scale (NRS), although pain is an intrinsic component in many measures. Health related quality of life measures, namely the Short Form Medical Outcomes Study Questionnaires (SF36/12), were next in frequency (6/29). Otherwise, there was a mixture of disability/functional recovery measures such as the Roland Morris Disability Questionnaire (RMDQ), Sickness Impact Profile (SIP) or Neck Disability Index (NDI). In addition, Post Traumatic Stress Disorder (PTSD) questionnaires were used in two studies [36,44]. Time to claim closure was used as a proxy health outcome in one study with other health and compensation related measures as predictors [43]. This study was included because time to claim closure represented a measure of recovery. Further, incorporating this study did not alter any conclusions. Taking into account the inclusion criterion of a 'validated health related outcome measure', most studies scored well (criteria O1-3) with 22/29 studies receiving full marks. Although two studies measured outcomes with face validity, rather than construct and/ or criterion validity [50,57].

Summary of other prognostic factors
Our search strategy was designed to only include studies that measured compensation related factors alongside other prognostic factors; therefore it was beyond the scope to report on all significant prognostic factors (these are listed in Table 3). Nevertheless, it is pertinent to provide some commentary.
Blame was a potential compensation related factor but it was described as 'external attributions of responsibility' or 'blaming' someone including themselves or work for their injury, which would not automatically mean access to compensation [4,53,64]. Hence, blame was excluded.

Grading of evidence
The association between each compensation related factor and health outcome is presented in Table 5. There was either a negative association or no association between a compensation related factor and the outcome measured. There were no reported positive associations, that is: no studies reported that compensation related factors were associated with improved health outcomes. The grades of evidence are determined with reference to Table 2.
A number of studies measured the association between two compensation related factors and an outcome; in most cases one predictor was significant and the other not significant [4,9,48,49,54]. Compensation related factors have the potential to be highly correlated. One of main objectives of this review was to determine the effect of each compensation related factor independently on an outcome. To avoid collinearity the non-statistically significant predictors were not considered and excluded from Table 5. Furthermore, the association varied depending on the outcome measured in seven studies [9,35,44,47,53,54,59,63].

Compensation related factors
Compensation status. The association between compensation status (Y/N) and poorer physical function was statistically significant in eleven studies (four high quality studies, three moderate quality studies and four low quality studies), and not statistically significant in seven studies (three high quality studies, three moderate quality studies and one low quality study). The association between compensation status (Y/N) and poorer psychological function was statistically significant in four studies (two high quality studies, one moderate quality study and one low quality study). The association between compensation status (Y/N) and increased pain was statistically significant in eight studies (two high quality studies, three moderate quality studies and three low quality studies), and not statistically significant in four studies (two high quality studies, one moderate quality study and one low quality study).
Legal representation. The association between lawyer involved (Y/N) and poorer physical function was statistically significant in five studies (three high quality studies and two moderate quality studies), and not statistically significant in two studies (one high quality study and one moderate quality study). The association between lawyer involved (Y/N) and poorer psychological function was statistically significant in three studies (two high quality studies and one moderate quality study).
Other compensation related factors. The association between receiving compensation at two years and poorer physical function was statistically significant in one high quality study.
The association between number of sick days in the three years prior to injury and poorer physical function was statistically significant in one low quality study. The association between number of sick days in prior three years and increased pain was statistically significant in one low quality study.
The association between claim type (having a claim under a specific scheme jurisdiction) and poorer physical function was not statistically significant in one moderate quality study. The association between claim type and increased pain was statistically significant in one moderate quality study and not statistically significant in one moderate quality study. The association between prior claim and poorer physical function was statistically significant in one moderate quality study. The association between prior claim and increased pain was statistically significant in one moderate quality study.
The association between tort insurance arrangements (as compared to no fault arrangements) and poorer physical function was statistically significant in one high quality study.

Strength of evidence recommendations
There is limited guidance to interpret these mixed results. GRADE refers to the inconsistency of relative treatment effects in binary/dichotomous outcomes following quantitative analysis. Inconsistency is described as a combination of negative and positive associations [66]. Following a review of the literature and consultation with experts, the level of evidence was downgraded for compensation related factors that showed both associations with poorer outcomes and no associations with an outcome [26,27,66]. Therefore, the evidence was downgraded for compensation status and poorer physical function; and compensation status and increased pain.
There is moderate evidence of an association between compensation status (having a claim) and poorer physical function. There is strong evidence of an association between compensation status and poorer psychological function. There is limited evidence of an association between compensation status and increased pain.
There is strong evidence of an association between legal representation (having a lawyer) and poorer physical function. There is moderate evidence of an association between legal representation and poorer psychological function.
There is moderate evidence of an association between receiving compensation at two years and poorer physical function. There is limited evidence of an association between number of sick days in prior three years, prior claim, and poorer physical function. There is limited evidence of an association between number of sick days in prior three years, prior claim, and increased pain. There is moderate evidence of an association between tort insurance arrangements and poorer physical function. There is limited evidence of no association between claim type and poorer physical function. There is inconsistent evidence between claim type and increased pain. The evidence levels are summarised in Table 6.

Discussion
This systematic review has focussed on identifying compensation related factors associated with health outcomes following musculoskeletal injury. A total of 29 studies were assessed with explicit categories for prognostic factors and health outcomes. Our results show that there is evidence of an association between different compensation related factors, predominantly compensation status (having a claim) and legal representation (having a lawyer), and poorer physical function; poorer psychological function; and increased pain following injury.
The strength of evidence varied according which compensation related factor and outcome were measured. This has been found by others when categorising results [25]. Mostly reviews focus on one outcome such as return to work or pain, or combine outcomes into functional recovery [11,13,14,33,67,68]. It is less common to separately classify outcomes. Nevertheless, we believe this provides more comprehensive results, and offers greater potential for comparison with future studies.
Our findings are consistent with other reviews that investigated the association between compensation related factors and health outcomes following whiplash and acute orthopaedic trauma [11,67,69]. Poorer outcomes have also been found for compensable patients following surgery [16]. All these reviews classified compensation related factors separately. Reviews with a generic classification tended to find no association [12,14]. In other research adversarial scheme design: fault versus no-fault; lack of early intervention; and longer claims duration were linked to poorer outcomes [6,7,70].
In a systematic meta-review, the authors concluded that evidence of an association between compensation related factors and health was unclear [19]. They referred to poor quality primary studies; proxy health outcomes; and heterogeneous compensation related factors. We have endeavoured to address these issues in our review. Comparable results were found in a whiplash review where over half the studies (9/16) reported an association between compensation related factors and poorer health outcomes, the remaining studies showed no association [18]. Studies finding an association between compensation related factors and poorer health outcomes were of similar quality to those that reported no association. Although the assessment methods were similar to ours: only whiplash injuries were selected; retrospective studies were included; outcome measures were not separated; and no scores were calculated. In addition, the authors questioned the validity of the results due to the potential for bias due to reverse causality.
There were two key factors, compensation status and legal representation, with a similar proportion of high and moderate quality studies that did and did not find a statistically significant difference in the association between these factors and the outcomes of physical function and pain. It is difficult to determine the reason for the disparate findings between studies. Study characteristics, including population, sample size, outcome measures and compensation scheme design were comparable in studies with a significant association and those with a nonsignificant association. The evidence for compensation status was downgraded when there was evidence of inconsistency, and data extraction and quality assessment methods were based on recommended criteria [21][22][23]28].
The strong and moderate levels of evidence between the compensation related factors of compensation status and legal representation, and poor psychological function following musculoskeletal injury, is not surprising. There has been growing evidence that involvement in a compensation process is stressful [71][72][73]. Recently, researchers found that many participants experienced high levels of stress during the claims process, and although poor health and vulnerability to stress played a role, it did not entirely explain the high levels of disability and poor psychological function post injury [74]. Similarly, these results were mirrored in a meta-analysis investigating the effect of compensation on mental health, which concluded that despite poorer mental health at baseline compensable participants did not improve as readily as noncompensable [15]. These findings lend weight to the apparent influence of compensation systems on poor psychological function particularly in the presence of poor baseline health measures.
In respect of reverse causality bias, although evidence exists of a correlation between claiming compensation and poor health, it is difficult to determine to what extent this is a casual relationship. Does claiming compensation cause poor health or does poor health lead people to claim compensation? Evidence to date suggests it occurs in tandem [15,74]. In our review two studies tested this hypothesis and found a difference in general health status between compensable and non-compensable participants at baseline and follow up [9,47]. Of the studies (13/ 29) that measured pre-injury and/or general baseline general health, six found that these variables were predictive of injury recovery [3,35,48,49,54,63]. We cannot refute the possibility of bias due to reverse causality based on our results.

Limitations
An important strength of this review was its conduct according to current guidelines and recommended methods of reporting [22][23][24][26][27][28]. Notwithstanding that, potential studies could have been missed because our search strategy focused on compensation wording in the abstracts. This was mitigated by hand searching of references, personal communication with experts, plus the authors' existing knowledge of papers to increase the likelihood of including of all relevant papers.
Another limitation was potential measurement error, which is likely when the timing of exposure to a compensation related factor does not occur at baseline. Possible reasons for this include: legislated time periods to lodge a claim; people choosing to submit a claim only if they are not recovering; timing of legal representation; and the interaction between eligibility to claim and different follow up periods. Some authors have chosen not to include compensation status because of the difficulty defining it as a baseline measure [13]. We felt it was impractical to exclude certain compensation related factors and/or studies on this basis. Moreover, definitions of baseline tend to vary between studies.
Interpretation of statistical results was also hindered by selective reporting, particularly poor explanations for final predictive models. Although this would not have changed our conclusions we were not able to explore the reasons behind particular associations.

Implications for policy and future research
Considering the number of studies investigating outcomes following musculoskeletal injury it is of concern that many do not include compensation related factors as a potential confounder given the evidence available. Compensation schemes are diverse and contextual which makes interpreting the evidence based on existing data classifications challenging. The development of a compensable reporting framework would be valuable and has been recommended by others [10,18,75,76]. Minimum reporting should include claim lodgement, claim acceptance, claim type, legal representation, entitlements, claim duration, litigation, sick leave, and weekly benefits paid for time off work if applicable. The timing of measures should be documented. For example: when legal representation or claim acceptance was obtained. A description should be provided of the legislative framework. Collaboration between researchers and the legal profession may also assist to untangle the complexities of scheme design particularly for future policy relevant research between and within jurisdictions [76,77].
It is imperative for researchers to consider reverse causality bias [18,78]. If present, this could be mitigated by risk assessments to identify triggers for poor recovery and facilitate early intervention. Furthermore, reducing compensation related psychological stressors such as: poor claims information and management; claim delays; perceived injustice; and numerous medico-legal assessments could improve injury recovery [74,79,80]. These stressors have also been linked to increased legal representation, delayed claim settlement and increased health care utilisation [15,71,73,81].

Conclusion
This systematic review demonstrates that there is evidence of an association between compensation related factors and poorer health following musculoskeletal injury. The evidence of whether this association is causal is less certain and further research is required. There is a definite need to compare baseline characteristics of compensable and non-compensable study populations and identify plausible reasons why compensation related factors are associated with poorer health.