Challenging the Norm: Further Psychometric Investigation of the Neck Disability Index

Accepted Manuscript

Challenging the Norm: Further Psychometric Investigation of the Neck Disability Index

Man Hung, PhD Christine Cheng, Shirley D. Hon, Jeremy D. Franklin, MA Brandon D.Lawrence, MD Ashley Neese, BS Chase B. Grover, Darrel S. Brodke, MD

PII: S1529-9430(14)00301-5

DOI: 10.1016/j.spinee.2014.03.027

Reference: SPINEE 55822

To appear in: The Spine Journal

Received Date: 11 July 2013

Revised Date: 12 February 2014

Accepted Date: 16 March 2014

Please cite this article as: Hung M, Cheng C, Hon SD, Franklin JD, Lawrence BD, Neese A, Grover CB,Brodke DS, Challenging the Norm: Further Psychometric Investigation of the Neck Disability Index, TheSpine Journal (2014), doi: 10.1016/j.spinee.2014.03.027.

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http://dx.doi.org/10.1016/j.spinee.2014.03.027

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Challenging the Norm: Further Psychometric Investigation of the Neck Disability Index

Man Hung, PhD Assistant Professor

University of Utah School of Medicine Huntsman Cancer Institute

Christine Cheng

University of Utah School of Medicine

Shirley D. Hon University of Utah College of Engineering

Jeremy D. Franklin, MA

University of Utah College of Education

Brandon D. Lawrence, MD Assistant Professor


Ashley Neese, BS University of Utah School of Medicine

Chase B. Grover


Darrel S. Brodke, MD Professor


Corresponding Author:

Man Hung, PhD Assistant Professor 590 Wakara Way, Salt Lake City, UT. 84108, USA Department of Orthopaedic Surgery Operations University of Utah School of Medicine Email: [email protected] Phone: 801-587-5372 Fax: 801-587-5411

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Challenging the Norm: Further Psychometric Investigation of the Neck Disability Index 1

2

3

ABSTRACT 4

5

BACKGROUND CONTEXT The Neck Disability Index (NDI) was the first patient-reported 6

outcome (PRO) instrument specific to patients with neck pain and it remains one of the most 7

widely used PROs for the neck population. The NDI is an appealing measure as it is a short and a 8

well-known PRO measure. Currently, there are conflicting data on the performance and 9

applicability of the NDI in patients undergoing either operative or non-operative treatment for 10

neck related conditions. 11

12

PURPOSE This study investigates the psychometric properties, performance, and applicability 13

of the NDI in the spine patient population. 14

15

STUDY DESIGN A total of 865 patients visiting a university-based spine clinic with neck 16

complaints, with or without radiating upper extremity pain, numbness or weakness were enrolled 17

in the study. Visit types included new and follow-up visits to both operative and non-operative 18

treatments. Questionnaires were administered electronically on a tablet computer and all patients 19

answered all 10 questions of the NDI. 20

21

METHODS Standard descriptive statistics were performed to describe the demographic 22

characteristics of the patients. Rasch modeling was applied to examine the psychometric 23

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properties of the NDI. 1

2

RESULTS The NDI demonstrated insufficient unidimensionality (i.e., unexplained variance 3

after accounting for the first dimension = 9.4%). Person reliability was 0.85 and item reliability 4

was 1.00 for the NDI. The overall item fit for the NDI was good with an outfit mean square of 5

1.03. The NDI had a floor effect of 35.5% and ceiling effect of 4.6%. The raw score to measure 6

correlation of the NDI was 0.019. 7

8

CONCLUSIONS Although the NDI had good person reliability and item reliability, it did not 9

demonstrate strong evidence of unidimensionality. The NDI exhibited a very large floor effect. 10

Due to the poor raw score to measure correlation, the sum score should not be used in 11

interpretation of findings. Despite great investment by physicians and other stakeholders in the 12

NDI, this evaluation and previous research had demonstrated that the NDI needs further 13

investigation and refinement. 14

15

16

17

18

Keywords: NDI; spine; patient-reported outcomes; measurement; Rasch; orthopaedics. 19

20

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22

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INTRODUCTION 1

2

In 2006, nearly 22 million people in the United States sought treatment for spinal disorders (1). 3

Physicians have stressed the importance of more robust measures to assess the condition of 4

patients with spine disorders. Traditionally, clinicians have relied on technology and clinical 5

measures to assess patients. Recently, the perspective of the patient or how they feel has become 6

a major interest of health care systems, organizations, and physicians themselves. As a result, 7

physicians are taking into account patient-reported outcome measures (PRO) when assessing 8

treatment success (2, 3). As in other fields, instrument use varies from physician to physician. 9

Furthermore, it is not clear which measures most accurately evaluate treatment effects for 10

patients with spinal disorders. In order to understand the comprehensive condition of their 11

patients, physicians need to incorporate valid and reliable measures. In conjunction with clinical 12

measures, the perceptions and perspectives of the patient need to be clearly understood in order 13

to identify appropriate treatments. 14

15

The Neck Disability Index (NDI), also known as the Vernon Mior Disability Index, was first 16

published in 1991, and is comprised of 10 items \ (See Appendix 1) (4, 5). As one of the first 17

PRO instruments specific to patients with neck pain., it remainsthe most widely used PROs for 18

patients with neck disorders and has been previously tested for validity and reliability (6, 7). It 19

has been shown to have construct validity and reasonable test-retest reliability (6). It is also 20

assumed to be a unidimensional scale (8, 9), although past research has disputed this (10). The 21

NDI instrument is appealing to physicians and patients because it isrelatively easy and quick to 22

administer.. The NDI is utilized throughout the world and has been translated into 19 languages 23

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(11). Although it has been demonstrated to be valid, some research had raised questions about 1

the performance and applicability of the NDI in patients undergoing either operative or non-2

operative treatment for their neck related condition. Furthermore, studies have investigated the 3

psychometric properties of the NDI and have found problems with ceiling effects, floor effects 4

and dimensionality (10-14). A systematic review of published studies on the NDI found 5

evidence of contrasting measurement properties of the NDI (11). While the majority of research 6

examining the NDI has used classical test theory (5, 8, 11), this study seeks to utilize 7

contemporary testing techniques, such as Rasch analysis, to further investigate the psychometric 8

properties, performance, and applicability of the NDI in the cervical spine patient population. 9

10

METHODS 11

12

Data Collection 13

A total of 865 patients visiting a university-based spine clinic from June 2011 to May 2013 were 14

asked to complete the NDI, a demographic questionnaire, and some outcome questions. Patient 15

visits were due to primary neck complaints, with or without radiating upper extremity pain, 16

numbness or weakness. Both new and follow-up patients with operative and non-operative 17

treatments were included in the final analysis. Questionnaires were administered electronically 18

on a tablet computer (iPad™, Apple Inc., Cupertino, CA) prior to seeing the physician. The 19

response rate was 100%as taking it is a standard of care measurement in this specific clinic. 20

Patients under 18 years old and non-English speakers were excluded. 21

22

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This was a self-funded study and none of the participants received any compensation. Informed 1

consent was not necessary since responding to the NDI was part of standard care in the clinic. 2

However, Institutional Review Board approval was obtained prior to data analysis. 3

4

Analytic approach 5

Descriptive statistics were conducted to examine the characteristics of the participants. Next, a 6

Rasch item response theory (IRT) model was utilized to assess the psychometric properties of the 7

NDI including the fit, dimensionality, reliability, coverage, and raw score to measure correlation. 8

The Rasch Partial Credit Model (PCM) for polytomous data was performed in this study using 9

the Winsteps software version 3.80.0. (15) Winsteps implements the PCM with the Joint 10

Maximum Likelihood Estimation method, also known as the Unconditional Maximum 11

Likelihood Estimation, which allows estimation of both the item difficulties and person abilities 12

simultaneously and does not assume any person distribution. (16-18) The PCM utilized in 13

Winsteps models each item with its own structure using the following formulation: log (Pnij / 14

Pni(j-1) ) = Bn - Di -Fij, where Pnj is the probability that person n responding to item I is observed 15

in response category j, Bn is the ability measure of person n, Di is the item difficulty measure of 16

item i, Fij is the calibration measure of category j relative to response category j-1 for item i (15). 17

The Rasch model assumes that item difficulty is the main characteristics affecting person 18

responses. Additionally, it produces person and item estimates along a logit scale, which 19

represents a unit interval scale. 20

The Rasch model represents an attractive approach for constructing instruments for 21

measurement. Specifically, it enables both the items and the persons to be measured on the same 22

metric, allowing for meaningful comparison of scores. It also enables “transformation on the 23

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item and person data to convert the ordinal data to yield interval data.” p. 29 (19) Prior to 1

employing the Rasch model to evaluate measurement properties, fit of the data to the model must 2

be established. (20, 21) For details into how a Rasch model works and technical terms as 3

defined by Rasch methodology, readers are encouraged to review references. (10, 20-24) 4

5

Fit 6

To evaluate the psychometric properties of the NDI we investigated the fit of the data to a Rasch 7

model. To indicate whether the data fit the Rasch model, we reported the outfit Mean Square 8

(MNSQ) statistic. We did not report infit MNSQ since infit MNSQ, as oppose to out MNSQ, 9

does not take the entire range of persons and items into account and has demonstrated to be 10

sample size dependent. (25) Fit is important because without adequate fit the Rasch model we 11

derive may not be applicable for the data. Data is considered to fit the model if the outfit MNSQ 12

is less than 2. (20, 21, 26, 27) Although MNSQ in between 1.5 and 2.0 generally implies the 13

model is unproductive for measurement construction, it does not distort measurement 14

nevertheless. 15

16

Dimensionality 17

A scale is considered to be unidimensional when it measures one underlying construct or type of 18

phenomenon, such as neck disability. If an instrument measures more than one underlying 19

construct (e.g. physical function, depression, attitude toward physician, etc.), then it is 20

multidimensional. As a result, a single summary score can be difficult to interpret because it is 21

unclear how to attribute proportions of the score to each of the multiple constructs. To evaluate 22

the unidimensionality of the NDI, a Rasch IRT model was used. (23) We evaluated the 23

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interrelationship of items within the NDI. If unexplained (residual) variance or noise is 5% or 1

less after taking account of the first dimension, the NDI is considered to represent a single 2

underlying concept (21). Unlike variance decomposition in classical test theory, the variance 3

components in Rasch model consists of those explained by the items as well as those explained 4

by the persons. The total unexplained (=residual) variance includes Rasch predicted randomness 5

and unexplained variance from the first dimension, second dimension, third dimension, etc. 6

Reliability 7

Additionally, we evaluated the internal consistency reliability of the NDI. In doing so, we 8

investigated person and item reliabilities. Person reliability is a measure of how reproducible the 9

ordering of patients is across items of the instrument. Item reliability is a measure of how 10

reproducible item ordering is across persons. Internal consistency is written as an r coefficient 11

that is between 0 to 1, where 0 indicates the least consistency and 1 indicates the most 12

consistency. (28). 13

Coverage 14

The coverage of an instrument is important when assessing its usefulness because instruments 15

should be applicable to all patients in the population of interest that have varying degrees of that 16

target condition. If it does not have good coverage it may not be able to distinguish among a 17

portion of the patient population. (29) If an instrument can measure the entire patient population 18

of interest then it is said to have very good coverage. (26) Ceiling effects and floor effects can 19

be used to assess instrument coverage. If an instrument is intended to measure disability, but is 20

not able to differentiate between patients with severe disability then it is said to have a ceiling 21

effect, that is, many persons will score at highest possible value. If patients are high functioning, 22

but the instrument is not sensitive enough to assess such granular nuances between their low 23

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levels of disability, then the instrument is said to have floor effect. If the entire patient 1

population is well targeted by the set of questions with minimal ceiling and floor effects, then the 2

instrument is considered to have excellent coverage. 3

4

Raw Score to Measure Correlation 5

Deriving a summary score by summing the values of all the items may not yield an interval scale 6

score, that is, one that can be used meaningfully in arithmetic operations. Without this property, 7

parametric statistical analyses (mean, standard deviation, t-test, ANOVA) are inappropriate. One 8

major advantage of Rasch modeling is that the Rasch model produces a summary of the items 9

that is an interval scale score. If the correlation between the raw score and the Rasch derived 10

measure is very high, it would mean that the instrument’s raw score behaves similarly to an 11

interval scale score and it would be appropriate to perform standard parametric analyses with the 12

raw score. On the other hand, it would be inappropriate to utilize the raw score if thecorrelation 13

is very low as it would signify that the raw score has little or no resemblance to an interval scale 14

score. 15

16

The raw score for NDI is the total score (i.e., the sum score from the ordinal response score with 17

a maximum of 50 multiplied by 2 to get a percentage. The correlation between the raw score and 18

the measure should be high (e.g., r = 0.7 or higher) for the raw scores to be meaningful and 19

useful in parametric statistical analyses. 20

21

22

RESULTS 23

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1

Demographics 2

A total of 865 patients made up the final sample with 57.5% male (Table 1). The majority of 3

participants were white (94%) and the mean age was 55 years old (ranging from 15 to 92 years 4

old). For return visit (i.e., follow-up visit) patients, about 49% stated that their pain/symptoms 5

were somewhat better or markedly better since their last visit. Close to half of the respondents 6

(46%) stated that they had two or more problems (e.g. balance, numbness, weakness). Almost 7

35% of the respondents tried multiple treatment methods to treat their pain. About 71% of the 8

participants stated that the primary reason for their visit that day was neck problems. When 9

asked about the percentage of their pain either being in their back and/or neck and percentage of 10

leg and/or arm pain, 5% of patients reported only arm pain and 16% reported only neck pain. 11

About 63% reported a combination of back and/neck pain and leg and/or arm pain. About 13% 12

of the participants stated their symptoms had bothered them for 1 to 3 months and about 23% 13

reported that their symptoms had bothered them more than 24 months. 14

15

Item level statistics 16

The NDI consists of a total of 10 items with its response categories ranging from 0 (least 17

disabled) to 5 (most disabled). Category 0 was selected most often (27.6% on average for all 18

items, which represents strong floor effect) and category 5 was selected least often (3.8% on 19

average for all items). The average point biserial correlations of the items were all positive 20

(ranging from 0.53 to 0.75) indicating that a high score on an individual item was related to a 21

high score for the entire instrument. The outfit MNSQ for the items ranged from 0.68 to 1.70 22

with a mean of 1.03 (std. dev. = 0.28). 23

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1

Fit 2

The overall item fit for the NDI was very good with a mean outfit MNSQ of 1.03 indicating the 3

data fit the Rasch model very well. Due to the close fit of the data to the model, it was 4

appropriate to move on with evaluating the properties of the NDI. 5

6

Dimensionality 7

The NDI demonstrated poor unidimensionality with unexplained variance in the first dimension 8

being 9.4%. Item 2 and 5 were largely misfit to the model with fit residual values being 4.55 and 9

4.67 respectively. Considering the NDI is supposed to measure one condition, we expected a 10

lower percentage of unexplained variance. The unexplained variance is high and this suggests 11

that the NDI may not be unidimensional in measuring neck disability. Nonetheless, this 12

departure from unidimensionality was not large enough (e.g., less than 10% unexplained 13

variance) to cause huge issues preventing further assessment and interpretation of of the persona 14

and item estimates using Rasch analysis. (30) 15

16

Reliability 17

The person reliability for the NDI was 0.85. This reliability suggests that similar ordering of 18

patients’ functioning levels would occur in repeated studies. The Rasch analysis indicated that 19

item reliability for the NDI was 1.00. An item reliability this high suggests that order of item 20

difficulty would be similar regardless of patient population or neck disability ailment. 21

22

Coverage 23

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Figure 1 shows a person item histogram. The scores shown on figure 1 were in interval scale, not 1

ordinal scale. The top panel is the distribution of the patient’s measures and the bottom panel is 2

the distribution of the item’s measures (i.e., item difficulties). The horizontal axis represents the 3

measures (in z score scale) where the left side indicates higher disability levels and right side 4

indicate lower disability levels. The vertical axis on the top panel represents the number of 5

patients and the bottom panel represents the number of items. In the figure, there is a large region 6

to the left of the person distribution not aligned with (or targeted by) the item distribution. 7

8

The NDI had a floor effect of 35.5% and ceiling effect of 4.6%. The floor effect of the NDI is 9

extremely high and the ceiling effect is acceptable. This implies that the items of the NDI did not 10

target patients with lower levels of disability of their neck. 11

12

Raw Score to Measure Correlation 13

Figure 2 displays a scatter plot of the NDI’s raw scores on the x-axis and the Rasch derived 14

measures on the y-axis. As demonstrated by the flattened fitted line across all the measurement 15

points, there is little to no correlation between the raw scores and the measures of the NDI, The 16

raw score to measure correlation of the NDI was 0.019. This correlation is extremely low, 17

implying that the sum score (raw score) is not interval scaled. 18

19

20

DISCUSSION 21

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Prior to conducting this study we were aware that that NDI was one of the most widely used 1

instruments to assess neck pain. A majority of the research we reviewed on the NDI found that it 2

was a reliable and valid instrument, but there are reported problems with ceiling effects, floor 3

effects, and lack of support for unidimensionality (10). Given the conflicting nature of the 4

current literature and ubiquitous use of the NDI, we felt the NDI required expanded 5

investigation. Our findings confirm previous critiques of the NDI, in that there are serious floor 6

effects and the instrument lacks strong unidimensionality. 7

8

Although the NDI exhibited good person and item reliabilities, it demonstrated marginal ceiling 9

effects and very poor floor effects. In this study, ceiling effects were 4.6% and floor effects were 10

35.5%, rates that are generally considered unacceptable. While this was not totally unexpected 11

given previous findings, it was still surprising considering the NDI is so commonly used to 12

assess patients with varying levels of disability. As a result, some patients may not be properly 13

assessed with the NDI. The high floor effect indicates that the NDI is a very poor instrument for 14

measuring patients who are moderate to high functioning and may not be sensitive to change in 15

patients’ functioning over time. 16

17

The raw score to measure correlation was poor, indicating that summing of the raw score is not 18

acceptable or meaningful. The commonly used NDI raw score is not linear and should likely not 19

be used in interpretation of findings. Proper transformation of the NDI raw score into a linear 20

measure is needed prior to statistical analysis and interpretation, since most statistical procedures 21

(e.g., t-test, ANOVA, Pearson correlation) assume that the scores used for conducting statistical 22

analyses are linear measures. 23

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1

This analysis confirms previous research and adds to the concerns of NDI usage as a PRO 2

measure. Unfortunately, the NDI does not carry with it a clear interpretation of what a score 3

means. The NDI does not exhibit strong unidimensionality, has serious limitations in regards to 4

its floor effects, and has an unacceptable raw score to measure correlation. 5

6

Limitations and Future Research 7

This study exhibits some threats to external validity in that it cannot necessarily be generalizable. 8

The sample consisted of participants that actively chose to visit a university-based spine clinic. 9

Therefore, this study has a very targeted sample that has the means and time to visit a clinic. 10

Furthermore, the study only took place in one clinic. The majority of the sample is White and 11

female. As a result, the findings may not be generalizable to other racial/ethnic groups. Future 12

research needs to incorporate a more diverse sample. Finally, future research might look into 13

modifications of the NDI or identify other neck disability instruments that can perform better and 14

to be incorporated into standard of care measurement. As a result, this will allow for more 15

accurate and precise measurement of neck disability and thus, provide better care for those 16

suffering from neck disorders. 17

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FIGURE LEGEND 9

Table 1. Demographic characteristics (N=865). 10

Figure 1. Person item histogram. 11

Figure 2. NDI raw score to measure correlation. 12

Appendix 1.Neck Disability Index. 13

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Variables Min Max Mean (SD) n Percent Age 15.4 91.6 55.2 (15.9) Gender Male 497 57.5 Female 368 42.5 Race White or Caucasian 784 94.0 Black or African American 4 0.5 American Indian and Alaska Native 10 1.2 Native Hawaiian and Other Pacific Islander 9 1.1 Asian 7 0.8 Other 20 2.4 Missing 31 Symptoms duration Less than 1 month 103 14.7 1 – 3 months 143 20.4 3 – 6 months 104 14.8 6 – 24 months 150 21.4 More than 24 moths 202 28.8 Missing 163 Treatment since last visit Nothing 160 23.1 Surgery 39 5.6 Physical therapy 66 9.5 Medications 166 23.9 Injections 20 2.9 Two or more treatment methods 243 35.0 Missing 171

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Section Everyday Activity Possible Responses 1 Pain Intensity 0. I have no pain at the moment.

1. The pain is very mild at the moment. 2. The pain is moderate at the moment. 3. The pain is fairly severe at the moment. 4. The pain is very severe at the moment. 5. The pain is the worse imaginable at the moment.

2 Personal Care (Washing, dressing, etc.)

0. I can look after myself normally without causing extra pain. 1. I can look after myself normally but it causes me extra pain. 2. It is painful to look after myself and I am slow and careful. 3. I need help but manage most of my personal care 4. I need help every day in most aspects of self-care. 5. I do not get dressed, wash with difficulty and sty in bed.

3 Lifting 0. I can lift heavy weights without extra pain. 1. I can lift heavy weights but it gives extra pain. 2. Pain prevents me from lifting heavy weights off the floor, but I can manage if

they were conveniently positioned, e.g. on a table 3. Pain prevents me from lifting heavy weights off the floor, but I can manage

light to medium weights if they are conveniently positioned. 4. I can lift only very lightweights. 5. I cannot lift or carry anything at all.

4 Reading 0. I can read as much as I want to with no pain in my neck. 1. I can red as much as I want to with slight pain in my neck. 2. I can read as much as I want with moderate pain in my neck. 3. I can’t read as much as I want because of moderate pain in my neck. 4. I can hardly read at all because of severe pain in my neck. 5. I cannot read at all.

5 Headache 0. I have no headache at all. 1. I have slight headaches, which come infrequently. 2. I have moderate headaches, which come infrequently. 3. I have moderate headaches, which come frequently. 4. I have severe headaches, which come frequently. 5. I have headaches almost all the time.

6 Concentration 0. I can concentrate fully when I want to with no difficulty. 1. I can concentrate fully when I want to with slight difficulty. 2. I have a fair degree of difficulty in concentrating when I want to. 3. I have a lot of difficulty in concentrating when I want to. 4. I have a great deal of difficulty in concentration when I want to. 5. I cannot concentrate at all.

7 Work 0. I can do as much as I want. 1. I can only do my usual work but no more. 2. I can do most of usual work, but no more. 3. I cannot do my usual work. 4. I can hardly do any work at all. 5. I can’t do any work at all.

8 Driving 0. I can drive my car without any neck pain. 1. I can drive my car as long as I want with slight pain in my neck. 2. I can drive my car as long as I want with moderate pain in my neck. 3. I can’t drive my car as long as I want because of moderate pain in my neck. 4. I can hardly drive at all because of severe pain in my neck. 5. I can’t drive my car at all.

9 Sleeping 0. I have no trouble sleeping. 1. My sleep is slightly disturbed (less than 1 hour sleep loss). 2. My sleep is mildly disturbed (1-2 hour sleep loss.)

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3. My sleep is moderately disturbed (2-3 hours sleep loss). 4. My sleep is greatly disturbed (3-5 hours sleep loss). 5. My sleep is completely disturbed (5-7 hours sleep loss).

10 Recreation 0. I am able to engage in all my recreational activities with no neck pain at all. 1. I am able to engage in all my recreational activities with some pain in my

neck. 2. I am able to engage in most but not all of my usual recreational activities

because of pain in my neck. 3. I am able to engage in a few of my usual recreational activities because of

pain in my neck. 4. I can hardly do any recreational activities because of pain in my neck. 5. I can’t do any recreational activates at all.

Challenging the Norm: Further Psychometric Investigation of the Neck Disability Index

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