World Health Organisation Disability Assessment Schedule (WHODAS 2.0): development and validation of the Igbo version in patients with chronic low back pain

The leading cause of years lived with disability globally is low back pain (LBP). Chronic low back pain (CLBP) is responsible for the cost and disability associated with LBP, which is more devastating in low-income countries, particularly in rural Nigeria with one of the greatest global LBP burdens. No Igbo back pain specic measure captures remunerative/non-remunerative work outcomes. Disability measurement using these tools may not fully explain work-related disability and community participation, a limitation not evident in the World Health Organisation Disability Assessment Schedule (WHODAS 2.0). This study aimed to cross-culturally adapt the WHODAS 2.0 and validate it in rural and urban Nigerian populations with CLBP.


Introduction
Disability is an encompassing label for impairments, activity limitations and participation restrictions, which result from the interaction between a health condition, and contextual (environmental and personal) factors [1]. Disability is a multidimensional concept including body function (physiological and psychological) and structure (anatomy); activities and participation; and environmental factors [1]. According to the World Health Survey, the global adult (≥ 18 years) prevalence of disability is 15.6%; ranging from 11.8% in higher income countries to 18.0% in lower income countries most of which are African countries [2]. Among people aged ≥ 60 years, disability prevalence is much higher, ranging from 43.4% in lower income countries to 29.5% in higher income countries. These prevalence rates are likely grossly underestimated in Africa where only a few people who have severe disability are reported [2].
The leading cause of years lived with disability globally is low back pain (LBP) according to global burden of disease studies [3,4]. LBP is predominantly regarded as a chronic condition because one year after its rst onset nearly 80% of patients still have pain [5]. Most of the cost and disability associated with LBP is due to chronic low back pain (CLBP) [6]. These consequences can be more overwhelming in low income countries, particularly in rural Africa with high levels of poverty and poor health services.
Nigeria has one of the greatest burdens of LBP globally. The prevalence rate of LBP ranging between 72-85% in rural Nigeria [7], is much higher than the 39% in urban Nigeria [8]; and 14-51% in other African countries [9]. Furthermore, the point prevalence rate of 33-40% in Nigeria is greater than the 10-33% in high income countries including the UK, Canada and Belgium [10].
Measures for LBP disability are mostly self-reported due to their low cost and ease of administration including reduced patient burden and non-invasiveness. Moreover, assessment of disability through selfreport may be comparable to objective disability measurements [11] and is sometimes more reliable than objective assessments since disability constructs such as participation restriction, may be more directly measured subjectively through self-reports. In contrast, performance-based disability measures may be impairment focused overlooking other important dimensions of disability such as activity limitations and participation [12]. This is limiting as people with impairment may not experience disability, or do so at varying levels depending on personal, physical and social barriers/facilitators in different contexts.
There are several back pain speci c self-report measures, the most commonly used being the Roland-Morris Disability Questionnaire and the Oswestry Low Back Pain Disability Questionnaire. However, none of these measures cover remunerative or non-remunerative work outcomes, which are often regarded as an aspect of participation [13,14]. This implies that disability measurement using these tools may not fully explain work-related disability, community participation, and other domains of participation which are likely to be context speci c. This limitation is not evident in the World Health Organisation Disability Assessment Schedule (WHODAS 2.0), an international classi cation of functioning, disability and health (ICF) based generic disability measure. The measure has distinct activity and participation domains, that include work-related disability and community participation [15]. Therefore, the WHODAS 2.0 might be one of the best measures for assessing LBP disability -being the most supportive of the biopsychosocial disability model. However, the original WHODAS 2.0 is in English [15], making it di cult to use in clinical and epidemiological studies for low literate non-English speaking people in rural and urban Nigeria. Therefore, this study aims to translate, culturally adapt and investigate the validity and reliability of the Igbo version of the WHODAS 2.0 in rural and urban populations in Nigeria. The WHODAS 2.0 is a comprehensive measure that assesses disability within the ICF biopsychosocial model emphasizing the six domains of cognition, mobility, self-care, getting along with people, life activities and participation -including work-related disability [15]. The measure has good face and content validity, construct validity, internal consistency, test-retest reliability and responsiveness. The Cronbach's alpha ranges between 0.94 and 0.98; test-retest reliability ranges between 0.93 and 0.98; and sensitivity to change ranges between 0.46 and 1.38 [15]. The 36-item interviewer-administered version, with simple and complex scoring methods was used due to its relevance in populations with low literacy. Simple scoring involves assigning values "none" =1, "mild" =2 "moderate" =3, "severe" =4 and "extreme" =5, which are simply added up without weighting of individual items. However, this method may not be comparable across populations and conditions. Therefore, the complex scoring method was used in this paper. Complex scoring is an "item-response-theory" (IRT) based scoring that takes into consideration multiple levels of di culty for each item. It involves summing recoded item scores in each domain, summing all six domain scores, and converting the summary score into a metric ranging from 0 (no disability) to 100 (full disability) [15]. Igbo Roland Morris Disability Questionnaire (Igbo-RMDQ) RMDQ is a valid and reliable measure of self-reported LBP disability; and recommended as a core outcome measure for LBP clinical trials [12]. It is easy to administer, understood effortlessly, and is the best disability measure for population-based or primary care-based studies. The Igbo-RMDQ, adapted from the original RMDQ, is a twenty-four item back speci c self-report disability measure with possible scores of 0 or 1 for each item. A score of 24 is the highest possible disability level and 0 is no disability. It has good face and content validity, construct validity, internal consistency, test-retest reliability and responsiveness [12]. It has a global Cronbach's alpha score of 0.91; test-retest reliability of 0.84; and a 2-3-point change from baseline means clinical signi cance [12].

Back performance scale (BPS)
BPS is a back-speci c performance-based measure of mobility-related limitation that is scored by external evaluator [16]. The instructions are simple and involves instructing participants to perform ve physical performance tests (sock test, pick-up test, roll-up test, nger-tip-to-oor test and lift test) involving mobility of the trunk. Sock test involves simulating putting on a sock normally from sitting. Pick-up test involves picking up a piece of paper from the oor normally. For the roll-up test, participant rolls up slowly from supine lying to long sitting with both arms relaxed. Finger-tip-to-oor test involves standing on the oor with feet 10 centimetres apart. There is then forward bending with straight knees and attempts to touch the oor with the ngertips. The distance between the oor and the ngertips is then measured in centimetres. The lift test involves a participant repeating the lifting of a 5-kilogram box from the oor to a 76 cm table and back to the oor for one minute, and the number of lifts recorded. Each test has scores between 0 and 3 depending on the di culty or ease with which they are performed.
A total score of 15 indicates maximum disability with 0 meaning no disability [16]. The BPS has a good internal consistency of 0.73, moderate correlations with self-reported disability (r = 0.454) con rming construct validity, and a good test-retest reliability of 0.91 [16,17].

Eleven-point box scale (BS-11)
As CLBP is painful and pain is a predictor of CLBP disability, pain intensity assessment was required. BS-11 is a single item eleven-point numeric scale of pain intensity [18]. It consists of eleven numbers (0 to 10) in boxes. Zero signi es 'no pain' and 10 is 'pain as bad as you can imagine' or 'worst pain imaginable'. The measure is easily understood among low literate populations in Nigeria [19].

Cross-cultural adaptation process
Translation is the linguistic rephrasing of a questionnaire into a different language whereas cross-cultural adaptation involves translation and cultural adaptation to enable the content validity of the instrument to be at a similar conceptual level in the new (intended) context [12].

Participants
A clinical physiotherapists with 18 years of clinical experience practising in Nigeria, three non-clinical translators (Igbo linguistic expert, business woman who studied linguistics and microbiologist who left Nigeria at 7 years old and grew to adulthood as a native English speaker in England), an expert review committee (English health psychologist and English academic physiotherapist working in the United Kingdom; Igbo clinical psychologist and Igbo clinical physiotherapist working in Nigeria), and a convenience sample of 12 adults living with non-speci c CLBP in rural Nigeria who had participated in a previous study [19] and gave informed consent, were involved in the cross-cultural adaptation process.

Procedure
The original WHODAS 2.0 was translated into Igbo and culturally adapted using internationally recognised evidence-based guidelines [20,21].
First step -the WHODAS 2.0 was forward translated independently from English to Igbo by one bilingual clinical physiotherapist and one bilingual non-clinical professional translator (who is an Igbo linguistic expert) to obtain two Igbo versions: T1 and T2 respectively. The forward translators were both native Igbo speakers uent in English. For the physiotherapist, items were explained to ensure an understanding of the construct being assessed by the questionnaire in order to provide psychometric equivalence with the original WHODAS 2.0. For the non-clinical translator, items were not de ned to ensure that the language and expressions used in the translation re ected the lay language routinely employed by people in Igbo populations.
Second step -a discussion between the two forward translators, mediated by the bilingual (English and Igbo) lead author resulted in the synthesis of T1 and T2 to produce one Igbo version: T-12. The lead author compared the translations, noted and recorded all discrepancies.
Third step -the synthesized Igbo version (T-12) was back translated from Igbo to English by two back non-clinical translators blinded to the original WHODAS 2.0 to produce two back-translated English versions: BT1 and BT2. The back translators included the businesswoman who studied linguistics, and the microbiologist who grew up as a native English speaker in England. Back-translation provided the validation that the adapted WHODAS 2.0 was reproducing the meaning in the original WHODAS 2.0.
Fourth step -a pre-nal Igbo version of the WHODAS 2.0 was produced following several meetings of the expert review committee during which all translated versions of the measure (T1, T2, T-12, BT1 and BT2) were discussed, mediated by the lead author. The committee achieved semantic equivalence by exploring Igbo and English words of the same object to determine if they meant exact things, if the same terms could have several meanings, and if linguistic di culties were encountered during the translations. The committee accomplished experiential equivalence with the original measure by ascertaining that items were experienced in the same way in the Western and Igbo cultures. The committee established comparable conceptual meanings for the words in the instructions, items, and responses in Igbo and Western cultures [20]. The Igbo words used in the translations were simple enough to be understood by anyone regardless of literacy.
Fifth step -twelve people living with CLBP in a rural Nigerian community [19] pre-tested the pre-nal Igbo-WHODAS 2.0. The think-aloud cognitive interviewing procedure was used. This involved participants reading out each item and loudly verbalising their thoughts as they attempted to answer it. Participants nally stated if they did not understand any item, what they understood by each question, and the perceived meaning of their selected response(s). All responses were recorded verbatim. This procedure helped to maintain equivalence between the two settings ensuring face and content validity of the Igbo WHODAS 2.0.

Psychometric testing process
Participants (sample size calculation for test-retest reliability) A minimum sample size of 27 is required to detect an intra-class correlation coe cient of 0.9 and a maximum width of 0.23 for a 95% con dence interval. A study for examining test-retest reliability was conducted with a convenience sample of 50 adults with CLBP who had no underlying serious pathology, radiculopathy or spinal stenosis. The participants were aged between 18 and 69 years and were recruited from rural and urban communities in Enugu State, South-eastern Nigeria. Informed consent was duly obtained prior to participation in the study.

Participants (sample size calculation for construct validity)
A correlation coe cient of 0.2 at a level of 0.05 with a power of 80% would require a sample size of 194. In a dataset with several high factor loading scores (> 0.80), a sample size of 150 would be su cient for exploratory factor analysis (EFA). A representative random sample of 200 adults with CLBP were recruited from rural communities in Enugu State as part of a larger population-based study and discussed in detail therein [22]. They were screened to rule out underlying serious pathology, radiculopathy or spinal stenosis. Informed consent was obtained prior to participation in the study.

Procedure for psychometric testing
A signi cant proportion of rural dwellers in Nigeria are not literate. Therefore, community health workers (CHWs), the front line of rural Nigerian primary health care, were recruited and trained for intervieweradministration of the questionnaires. The training was aimed at minimising common survey errors. A representative sample of the population obtained through multistage cluster sampling prevented coverage error. An adequate sample size and gender strati cation prevented sampling error. The use of validated measures and training CHWs to avoid administering the measures in ways that could introduce prejudice to participants' responses reduced measurement error. Non-response error was avoided by ensuring that no items or scales were unanswered and that all recruited participants were assessed.
Collection and delity of data CHWs screened participants by asking simple questions to exclude back pain due to malignancy, spinal fracture, infection, in ammation or cauda equina syndrome. They were then asked to describe the location of their pain with a body chart to con rm that pain was in the lower back. The Igbo-WHODAS 2.0, Igbo-RMDQ and BS-11 were then interviewer-administered with Likert scales presented to participants as ' ash cards' as each item was read out. 'Back pain' was read out to participants in place of 'illness'. The BPS, which measures performance-based disability was then administered objectively.
For test-retest reliability, measures were completed at baseline and repeated seven to ten days postbaseline, with the same CHW collecting data on the two occasions from the 50 participants.
To test validity, measures were completed at one time-point cross-sectionally among the 200 participants.
Fidelity checks were done to avoid systematic differences in data collection. The CHWs were given posttraining examinations, and only those that passed them were involved in data collection. This facilitated adherence to data collection protocols. Additionally, each CHW was visited by the lead author during data collection without previous warning to assess their data collection and recording.
Data analyses IBM Statistical Package for Social Sciences version 22 (SPSS, Chicago, IL) was utilised. Normality of data was assessed using visual (normal distribution curve and Q-Q plot), and statistical (Kolmogorov-Smirnov, Shapiro-Wilk's test and Skewness/Kurtosis scores) methods.

Reliability
Reliability is the ability of an instrument to measure consistently. Test-retest reliability evaluated how consistently the adapted WHODAS 2.0 consistently measured disability over time using intra-class correlation coe cient (ICC). ICC was calculated using a two-way random effects model (measurement errors arising from either raters or subjects), using an absolute agreement de nition between test-retest scores. 0.7, 0.8 and 0.9 signi ed good, very good and excellent ICCs [23]. Internal consistency (Cronbach's alpha) depicts the extent to which all items in a test measure the same construct and was rated as weak (0-0.2), moderate (0.3 0.6) and strong (0.7-1.0) [24]. Bland-Altman plots, which accounted for the weakness of ICC that might indicate strong correlations between two measurements with minimal agreement, were employed to visually assess the agreement level between test-retest measurements by plotting mean scores against difference in total scores. Standard error of measurement (SEM) and minimal detectable change (MDC) were also used to investigate reliability. MDC is a statistical estimate of the smallest change detected by an instrument that corresponds to a noticeable change in ability which is not due to measurement error. MDC was calculated with the standard error of measurement (SEM), based on the distribution method, and the reliability of the measure which takes precision into account [25]. SEM was based on the standard deviation (SD) of the sample and the test-retest reliability (R) of the self-report measures, and was calculated with the equation [25] SEM = SD √(1-R) MDC was subsequently calculated with the equation [25]: MDC = 1.96 * √2 * SEM 1.96: 95% con dence interval of no change; √2: two assessments are involved in measuring change [25].

Validity
Construct validity assesses the extent to which a measure evaluates the construct it was intended to measure. The domain of construct validity assessed was convergent validity, which assesses whether two measures of the same/similar construct that are assumed to be theoretically related, are in fact related. This was investigated using Spearman's correlation (non-parametric data) and was rated as weak (0-0.2), moderate (0.3-0.6), and strong (0.7-1.0). The WHODAS 2.0 assesses self-reported disability within the ICF multiple domains of cognition, mobility, self-care, getting along with people, life activities and participation -including work-related disability. Hence, Igbo-WHODAS 2.0 is expected to correlate at least moderately with the Igbo-RMDQ (measuring self-reported back pain-related disability), the BPS (objective measure of performance-based disability), and the Igbo-BS-11 (self-reported pain intensity measure and a predictor of self-reported disability [22,26]. Exploratory factor analyses (EFA) was used to determine the number of factors in uencing the Igbo-WHODAS (the items that go together -dimensionality). EFA was applied in line with the Kaiser Meyer Olkin (KMO) and the Bartlett's test with eigenvalue for retention set at ⩾1.0 (Kaiser's rule) [27]. Retained and excluded factors were also explored visually on a Scree plot. Promax (oblique) rotation, which assumes that factors can be related, was done, and factor loadings less than 0.3 were suppressed. Extraction was done using principal axis factoring. The number of factors and the fundamental relationships between the items were then compared with the factor structures of the original WHODAS 2.0 to augment any insight of possible differences in population characteristics.

Floor and ceiling effects
When a high proportion of participants score the highest or the lowest score, ceiling or oor effect respectively occurs, which implies that a measure is unable to discriminate between either extreme of the scale. A ceiling or oor effect was de ned as 15% or more of the total sample of 250 participants scoring 0 or 100 on the Igbo-WHODAS 2.0 [28].

Results
Cross-cultural adaptation participants Table 1 highlights the socio-demographic characteristics of the participants that pre-tested the Igbo-WHODAS. Psychometric properties of Igbo-WHODAS Participants Table 2 presents the socio-demographic characteristics of the test-retest reliability sample. Table 3 presents the socio-demographic characteristics of the cross-sectional validity sample.

Findings from delity assessment
The CHWs strictly adhered to the interviewing styles recommended during the training by remaining neutral throughout the interviews. They did not react verbally or nonverbally to participants' responses.
They discouraged participants' digression, distraction and inappropriate enquiries. They maintained the wording and sequence of questions in the measures and recorded data as appropriate. They provided only one answer to each item, written in the space provided for each item in each measure. Their assessment of performance-based disability was adequate, as they used tape measures adequately to assess 10 cm between the feet and measured the distance between the ngertips and the oor, for the nger-tip-to-oor test. The performance-based disability levels recorded by the rst author and the CHWs were found to be similar for a random subsample of participants.

Reliability
Internal consistency was very good to excellent (α ≃ 0.8 to 0.9). Intraclass correlation coe cients were very good to excellent (ICC ≃ 0.8 to 0.9). Standard error of measurement and minimal detectable change were 5.05 and 13.99 for total scoring; 7.20 and 19.96 for cognition; 8.00 and 22.17 for mobility; 7.20 and 20.35 for self-care; 7.60 and 21.07 for getting along with people; 8.70 and 24.11 for life activities; 11.10 and 30.77 for participation. Acceptable agreements between test-retest values of the Igbo-WHODAS and its subscales were illustrated in Figs. 1 to 7 as mean differences were close to zero and most points were within the 95% limits of agreement of the mean differences (Table 4). Construct validity Table 5 illustrates the total scoring of the Igbo-WHODAS and its subscales which correlated moderately (rs ≥ 0.3) with performance-based disability (BPS), self-reported disability (Igbo-RMDQ), and pain intensity (BS-11), except for the cognition and getting along subscales. There was a weak (rs = 0.19) but statistically signi cant correlation between the cognition subscale of the Igbo-WHODAS and performance-based disability. There was no correlation between the getting along subscale of the Igbo-WHODAS and performance-based disability.
A scree plot in Fig. 8 suggests a seven-factor structure of the Igbo-WHODAS, which is corroborated in Table 6. 62.79% of the items had factor loadings above 0.5 and 66.67% of the items loaded on their corresponding factor in the original measure. Factor 1 contains all the items of the original life (household and work/school) activities subscale in addition to two items of the original participation subscale -problem joining in community activities (D6.1), and problem doing things by oneself for relaxation/pleasure (D6.8); and one item of the original self-care subscale (staying by oneself for a few days).   matches the self-care subscale of the original measure except for one missing item (staying by yourself for a few days) that loaded on the life activities factor. Factor 7 had only one major item (barriers and hindrances in the world around one due to back pain) from the original participation subscale (Table 6). However, oor effect was observed in cognition, self-care and getting along subscales.

Discussion
The exact Igbo word equivalents for some English words were lacking during the translation of the WHODAS 2.0 which was resolved by using Igbo phrases that retained the conceptual meaning in the original items. This could be because Igbo language may be more adapted to colloquial speech than scienti c writing [29]. Indeed, English is the o cial written language of instruction in Nigeria which may explain why literate Igbo Nigerians prefer to read/write English but speak Igbo informally. It was found that some Igbo words/phrases had multiple meanings depending on the context, which was resolved by using Igbo phrases with all possible meanings re ecting the original items.
The WHODAS 2.0 was straight forward to cross-culturally adapt, comprehend and was acceptable, as suggested by previous adaptations [30][31][32]. The cross-cultural adaptation con rmed its face and content validity. The lack of an Igbo word for 'emotion' in item D6.5 may re ect the unclear emotional concept in this culture where emotional distress is often expressed through somatisation [19,33], which has been found in other non-western settings [34,35]. 'Affected your heart or spirit' was therefore used to achieve conceptual equivalence.
Cronbach's alpha of Igbo-WHODAS and its subscales ranging between 0.75-0.97 concurs with the original measure [15], and other adaptations [36][37][38]. However, the Cronbach's alpha was slightly higher in the original measure possibly due to different population characteristics such as literacy.
Igbo-WHODAS and its subscales demonstrated reliability with ICCs that were very good to excellent (0.81-0.93). The good agreement shown in the Bland-Altman plots mirrors the original measure [15], and other adaptations [37,38].
Regarding the appropriateness of the SEM and MDC, 19% (Japan) to 51% (Nigeria) reduction in WHODAS is clinically important [15]. This corresponds to between 4.8 and 12.97 of Igbo-WHODAS mean of 25.44.
Therefore, SEM of 5.05, MDC of 13.99 and limits of agreement of -8.58 to 9.54 of Igbo-WHODAS appear suitable.
Igbo-WHODAS and its subscales correlated at least moderately (rs ≥ 0.3) with performance-based disability, self-reported back pain speci c disability (Igbo-RMDQ), and pain intensity (BS-11), except for the cognition and getting along subscales. There was a weak (rs = 0.19) but statistically signi cant correlation between the cognition subscale of the Igbo-WHODAS and performance-based disability. There was no correlation between the getting along subscale of the Igbo-WHODAS and performance-based disability. This lack of association could be because the getting along with people subscale of the Igbo-WHODAS appears to re ect the psychosocial aspect of the biopsychosocial disability model whereas the back-performance scale measures the biomedical aspect of the biopsychosocial disability model. In contrast to the Igbo-WHODAS which fully captures the multidimensional biopsychosocial disability concept including impairments, activity limitations and participation restrictions, performance-based disability is impairment focused. Impairment represents abnormalities or loss of body structure and function and conceptualises disability at the level of the body only [1]. Impairment does not automatically imply disability, as people with impairment may not experience disability, or do so at varying levels depending on personal, physical and social barriers/facilitators in different contexts [39]. Evidence suggests that performance-based disability characterise impairment-focused biomedical variables (e.g. leg strength, leg velocity), whereas patient-reported disability represent both impairment and psychosocial aspects of disability [40]. This agrees with our ndings showing the greatest correlations between Igbo-WHODAS, and its mobility, participation, and life activities subscales, with back pain speci c disability (Igbo-RMDQ) and pain intensity (BS-11) which are patient-reported outcomes. Furthermore, these subscales represent the construct within back pain speci c measures. Cognitive dysfunction may be less important than limitations in mobility, life activities (di culties in performing speci c actions, tasks or activities related to household activities and work/school activities) and participation (di culties of individuals to participate in community activities within speci c societies) in mobility-related disability in this population. As expected, the mobility subscale of the Igbo-WHODAS had one of the strongest correlations with the BPS which measures mobility-related disability [16]. These ndings support the construct validity of the Igbo-WHODAS 2.0.
A seven-factor solution of the Igbo-WHODAS was produced similar to adaptations in European languages [37] and Chinese [38]; in contrast to the six factors in the original measure [15]. Most Igbo-WHODAS items loaded on their corresponding factor in the original measure except for participation. The participation subscale of the original WHODAS 2.0 (meant to re ect the impact of participants' back pain on their participation in society) was the least precise with only two of the original eight items ('drain on nancial resources' and 'problem to family') loading on factor 5. The other items in the original participation subscale loaded on all other factors except self-care. Differences could be due to high illiteracy resulting in high measurement error or different population characteristics, although the latter is more likely to be the case.
Factor 1 of the Igbo-WHODAS can be termed life activities, community involvement and functional independence factor as it re ects the di culties participants may have in: performing daily household/work/school activities, joining in community activities, doing things or staying by oneself. The rural dwellers from whom the factor structure of the Igbo-WHODAS was derived were mostly involved in informal self-employed occupations within the community [12,19,22] which could explain why work activities, community involvement and staying/doing things for oneself loaded as one factor. Factor 2 of the Igbo-WHODAS can be retained as the getting along factor as in the original subscale. The additional loading of one item of the original participation subscale D6.3 and one item of the original cognition subscale D1.5 suggests that living with dignity due to the action of others and understanding what people say are key to people living with CLBP getting along with others in the community.
Factor 3 of the Igbo-WHODAS can be named mobility and concern factor since two additional items from the original participation subscale (time spent on [19] from this population suggests that reduction of nancial resources due to work-related disability from CLBP had a great negative effect on family relationships causing family problems as indicated by participant comments: "…It means that you are not able to do the work that supports your existence. With that you will see that there will be no money, there will be no food until I recover and start going to work...'' (P3, Male, aged 42 years). "…brings problems into the home...because the money isn't enough…"(P17, Male, aged 46 years) [19].
Factor 6 is entitled self-care as in the original self-care subscale despite having one missing item (staying by yourself for a few days D3.4) that loaded on factor 1 (life activities, community involvement and functional independence factor). Notably, this item D3.4 in the original self-care subscale appears very similar to item D6.8 problem doing things by oneself for relaxation/pleasure in the original participation subscale. These concepts appear to belong to one construct and should be examined in future studies.
Factor 7 can be seen as redundant as it had only one major item D6.2 (barriers and hindrances in the world around one due to back pain) from the original participation subscale. However, factor 7 had secondary loadings from two items, D6.3 (problem living with dignity due to attitudes/actions of others) and D4.2 (di culties maintaining a friendship), both of which loaded primarily on factor 2 (getting along with people). This suggests that the barriers and hindrances that people with CLBP in rural Nigeria face in the world around them could be related to problems they have living with dignity due to attitudes/actions of others and di culties maintaining a friendship. These ndings require further exploration. Moreover, further research is required to con rm the factor structure of the Igbo-WHODAS.
The Igbo-WHODAS 2.0 did not have oor and ceiling effects. However, the oor effects observed in the cognition, self-care and getting along subscales could also mean that these are not the major domains affected in CLBP-disability in rural Nigeria where emphasis appear to be on pain intensity, mobility, work activities and participation in society [19,22].
This study enabled the development of a valid and reliable generic measure of disability for Igbo speaking populations. This is important since non-English speaking rural Nigerians are often neglected clinically and during research despite having one of the highest disability rates. The demonstrated complexity of developing a valid and reliable measure for this population could be related to cultural, linguistic and literacy issues.
Despite acceptable validity and reliability levels, high sample variability and measurement errors were probably introduced by low literacy rates, interviewer-administration and data collection by several raters. This is important as MDC not only depends on the inherent measurement error of an instrument, but varies across populations and contexts [41,42]. Hence, sensitivity-to-change studies of the Igbo-WHODAS 2.0 is required in populations of varying literacy levels, with single raters, and using more rigorous analysis such as receiver operating characteristic (ROC) curves, which includes patients' own global impression of change [43]. Furthermore, these studies need to con rm the MDC of the Igbo-WHODAS, and determine the proportion of people that achieve it. Bilingual assessment of the agreement between the original WHODAS and Igbo-WHODAS 2.0, including item by item agreement in a population with adequate literacy levels to enable comprehension of the English and Igbo versions is also necessary.

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The lack of rigorous investigation of item redundancy in this study can be explored in future studies.
Redundancy could be demonstrated in terms of items that are too similar which spuriously in ate reliability [44], or items that are not applicable in this particular culture or population [45]. Reducing redundancy involves excluding items that are not applicable in a population following assessment by a team of content experts from a culture. Items rated by a single team member as irrelevant, or by two or more members as questionably relevant should be eliminated, whereas items obtaining one rating of questionable relevance should be reconsidered for inclusion. Re-assessment of internal consistency would then be needed when any item is removed from a measure to ensure that an acceptable Cronbach's alpha (> 0.60) is maintained [45]. Following the elimination of redundancy, multi-group con rmatory factor analyses may be needed to compare and determine the factor structures with the best t indices in rural Nigeria, assess if the same items assess the same construct in different populations in rural Nigeria, whether the items of a given factor are equally signi cant within different cultures in rural Nigeria or are too different; and if items are more biased towards some cultural groups than others. Using item response theory, items with different functioning may be eliminated so that groups are comparable, in which case the measure becomes somewhat different from the original or considered differently in separate groups to maintain equivalence between scores [44].
The acceptable internal consistency of the Igbo-WHODAS 2.0 suggest that items were su ciently independent but were adequately similar. However, Principal Components Analysis (PCA), a data reduction technique which identi es and discards highly correlated items may be required in future studies involving the Igbo-WHODAS 2.0. As PCA is a large sample evaluation requiring at least ve times the number of items in a questionnaire being analysed, a much larger sample size than the one used in this study will be required in future studies. This is more so when only a few items are expected to load onto each component, and when variable communalities (percentage of variance in an observed variable that is accounted for by the retained components) are low [46]. Furthermore, con rmatory factor analyses would require a sample size of at least 300 when there are only a few high factor loading scores (> 0.80) [47]. This should be the focus of future research.
Other strengths of this study include the validation of Igbo-WHODAS 2.0 with both self-reported and performance-based disability as well as pain intensity measures, with established correlations which are in line with the literature, supporting its validity.

Conclusions
The Igbo-WHODAS is valid and reliable for clinical and research purposes in Igbo speaking culture. It would support global health initiatives which often involve concurrent activities in countries of different languages and culture.