Background: Disrupted self-perception of the low back might contribute to chronic non-specific low back pain. The Fremantle back awareness questionnaire is a simple questionnaire to assess back specific self-perception. The questionnaire has recently been translated to German (FreBAQ-G). The aim was to further investigate the psychometric properties of the FreBAQ-G, to evaluate its cross cultural validity in patients with chronic non-specific LBP and to explore potential relationships between body perception, pain, disability and back pain beliefs. Methods: In this cross-sectional multicentre study, sample data were merged with data from the validation sample of the original English version to examine cross-cultural validity. Item Response Theory was used to explore psychometric properties and differential item function (DIF) to evaluate cross-cultural validity and item invariance. Correlations and multiple linear regression analyses were used to explore the relationship between altered back specific self- perception and back pain parameters. Results: Two hundred seventy-two people with chronic low back pain completed the questionnaires. The FreBAQ-G showed good internal consistency (Cronbach's alpha = 0.84), good overall reliability (r = 0.84) and weak to moderate scalability (Loevinger Hj between 0.34 and 0.48). The questionnaire showed unidimensional properties with factor loadings between 0.57 and 0.80 and at least moderate correlations (r > 0.35) with pain intensity, pain related disability and fear avoidance beliefs (FABQ total - and subscores). Item and test properties of the FreBAQ-G are given. Only item 7 showed uniform DIF indicating acceptable cross-cultural validity. Conclusions: Our results indicate that the FreBAQ-G is a suitable questionnaire to measure back specific self-perception, and has comparable properties to the English-language version.
Keywords: Low back pain; Disrupted self-perception; Body image; Item response theory; Validity
Supplementary Information The online version contains supplementary material available at https://doi.org/10.1186/s12891-021-04156-1.
Low back pain (LBP) is the leading cause for years lived with disability worldwide [[
While there is consensus that non-specific LBP is a multifaceted health problem with complex interactions between various biological, social and psychological factors [[
To assess back specific self-perception in persons with LBP, the Fremantle Back Awareness Questionnaire (FreBAQ) was recently developed [[
- To investigate the psychometric properties of the FreBAQ-G using Item Response Theory. Based on IRT modelling item functioning characteristics, such as item difficulty, discrimination and item information were examined. In addition the distribution of the items over the scale as well as test-score properties including reliability parameters and measurement error were evaluated. Finally item invariance of the FreBAQ-G was assessed to examine whether the questionnaire behaves in the same way in different subgroups of the German speaking population.
- To investigate cross-cultural validity / item invariance of the FreBAQ-G in patients with NSCLBP. Based on IRT techniques differential item functioning (DIF) was used to evaluate whether the translated version behaves in the same way in the German speaking population as the original version in the English speaking population.
- To investigate hypothesis based construct validity of the FreBAQ-G by evaluation of the correlations of back specific self-perception with other back pain related parameters such as pain intensity, function and fear avoidance beliefs corresponding to the English validation study [[
20 ]].
The study was designed as a multicentre, cross-sectional study. Data was collected as part of a study evaluating lumbar movement control in persons with NSCLBP. All participants provided written informed consent and all procedures conformed to the Declaration of Helsinki. To investigate cross-cultural validity the data of this study were pooled with those collected by Wand et al. [[
Participants were recruited in seven physiotherapy practices in Germany between April and September 2019.
Participants had to meet the following inclusion criteria: age ≥ 18 years; sufficient German language ability to complete the questionnaire; currently experiencing NSCLBP with or without leg pain (leg pain above the knee and main pain had to be localized below the costal margin and above the inferior gluteal folds) and duration of symptoms ≥3 months. The pain level, calculated as the mean of the actual pain intensity and the average pain intensity during the last 3 months, measured on an 11-point numeric rating scale (NRS), needed to be above 0. Participants were excluded if they had signs and symptoms indicating specific spinal pathology [[
Data collection and data analysis was conducted by different persons to minimize potential risk of bias.
Participants provided basic demographic information and completed a self-developed questionnaire to collect information about LBP characteristics. Pain related disability during daily activities, leisure time and work, as well as pain intensity, were assessed using 11-point numerical rating scales (NRS 0 = no pain / disability - 10 = worst imaginable pain / disability). For overall pain related disability we calculated the mean of the impairment scores during daily activities, leisure time and work. Pain related fear was estimated using the German version of the Fear Avoidance Beliefs Questionnaire (FABQ) [[
For questionnaires with ordinal scaled items, polytomous item response models are recommended [[
Descriptive statistics were used to describe the demographic and clinical characteristics of the sample. The FreBAQ-G was summarized using range, median, mean and standard deviation for the total score. The frequencies in each response category were also reported.
IRT modelling was used to assess cross cultural validity and the psychometric properties of the FreBAQ-G. Because the 9 items of the FreBAQ-G are ordinal scaled, a polytomous IRT model, should be used [[
Psychometric properties, including scalability, internal consistency, item characteristics, test characteristics and test reliability of the FreBAQ-G were calculated. Differential item functioning (DIF) was used to evaluate item invariance, which means whether different subgroups of the German speaking sample have the same chance to answer the items of the FreBAQ-G.
Internal consistency was estimated using Cronbach's α. Acceptable internal consistency is reached if α is > 0.7 [[
After fitting the GRM model, the test- and item-characteristics were evaluated. In IRT modelling, a person's ability in the latent trait -in this study" back specific self-perception"- is measured on a logit scale which follows a Z-distribution with a mean of 0 and a SD of 1 (range from − 4 to 4) [[
The test characteristic curve visualizes the relationship between the IRT-based estimated ability in the latent trait "back specific self-perception" for each person and the expected classical sum-score, based on the classical test theory [[
The test information function shows how precisely the FreBAQ-G can estimate the level of the respondent's ability in the latent trait [[
The item characteristics include item discrimination (slope), item difficulty (threshold) and item information [[
Finally, differential item function (DIF) was used to assess the assumption of item invariance [[
Cross cultural validity refers to the equivalence of measurement across different cultural groups [[
The relationship between the IRT-based estimated FreBAQ-G score (Theta) and pain intensity, disability and fear avoidance beliefs was calculated using correlation statistics (Pearsons r coefficient). Finally multiple linear regression with the FreBAQ-G (estimated with the Theta) as the dependent variable was performed to find the best predictors.
For statistical analyses Stata 16.1 (StataCorp LLC, USA) was used. The IRT model fit statistics was calculated using the student version of IRTPRO 4.2 (Scientific Software International Inc., USA).
Two hundred seventy-two patients with NSCLBP were included of which 271 completed all questionnaires. Table 1 gives a summary of the demographic data of this sample and of the sample used by Wand et al. [[
Table 1 Demographic data of participants of this study and the English language population collected by Wand et al. [[
German language sample (this study) English language sample Sample size n = 251 Age (years) mean (SD) 42.4 (±15.8) 48.8 (±13.4) Height (cm) mean (SD) 173.8 (±9.4) 170.9 (±9.8) Weight (kg) mean (SD) 75 (±16) 80.6 (±16.7) Body mass index (BMI), mean (SD) 24.75 (±4.4) 27.6 (±5.2) Sex female n (%) 167 (61.4) 148 (59.0) Average back pain intensity (SD) 3.8 (±2) 5.8 (±1.9) Fear avoidance (FABQ-PA scale) (SD) 9.6 (±5.8) 14.1 (±6) FreBAQ (SD) 8.0 (±6) 9.8 (±6.6)
SD Standarddeviation, FABQ-PA scale Fear avoidance beliefs questionnaire-subscale physical activity, FreBAQ Freemantle back awareness questionnaire
At the time of measurement, the average pain intensity within the last 3 months was 3.8 (SD 2) and the actual pain intensity was 3.4 (SD 2.1) measured on a 0–10 NRS. The average pain level was 3.6 (SD 1.7) and the average pain related disability was 3 (SD 2.1) on a 0–10 NRS.
The mean Fear Avoidance Beliefs Questionnaire (FABQ) score of all participants (N = 272) was 20.4 (SD = 12) (range 0–60, higher values indicating greater levels of fear avoidance). The mean value in the subscale physical activity (FABQ-PA) was 9.6 (SD 5.8) (range 0–24) and 10.8 (SD 9) in the work subscale (FABQ-W; range 0–42). At the time of measurement, 99 participants were receiving physiotherapy treatment.
The average total FreBAQ-G score was 8.0 (SD 6, range 0–27), with a median of 7.0 (interquartile range 3–12). Figure 1 shows the distribution of the FreBAQ-G sum-scores.
Graph: Fig. 1 Sumscore FreBAQ Range 0–36, higher values indicate greater levels of impaired back specific self-perception
Two hundred sixty-four subjects had no missing values, 6 subjects had 1 missing value each, 1 subject had 5 missing values and 1 subject had 9 missing values (see also Table 2).
Table 2 Frequencies and responses to each FreBAQ-G item
Item Miss-ing % N response categories in % Alpha item Loev Hj coeff Never feels like that (0) Rarely feels like that (1) Occ. or some of the times feel like that (2) Often, or a moderate amount of time feels like that (3) Always, or most of the time feels like that (4) Median Mean 1 0.74 270 49,89 24,44 19,26 6,67 0.74 1 0.85 0.82 0.43 2 1.10 269 28,62 29,37 25,65 13,38 2,97 1 1.3 0.83 0.42 3 0.74 270 47,41 31,11 15,56 5,56 0.37 1 0.79 0.82 0.45 4 0.74 270 41,85 28,15 17,04 11,48 1,48 1 1.01 0.82 0.48 5 1.47 268 43,28 25,37 17,16 11,19 2,99 1 1.04 0.82 0.47 6 1.10 269 60.97 23,42 7,43 6,32 1,86 0 0.64 0.82 0.45 7 0.37 271 71,22 14,76 9,59 4,43 – 0 0.47 0.83 0.39 8 0.74 270 79,63 12,59 5,93 1,85 – 0 0.29 0.84 0.39 9 0.37 271 25,46 22,14 26,20 20,30 5,90 2 1.57 0.84 0.34 Scale Cronbach alpha (interrelatedness) 0.84
The frequencies, median responses and missing values to each FreBAQ-G item are given in Table 2. Response option 4 (always or most of the time feels like that) was not chosen for items 7 and 8. For items 1 to 8, the distribution of the responses to each category is left skewed towards the option 0 (never feels like that), whereas for item 9 the responses are more equally distributed. Loevinger Hj coefficients are between 0.34 and 0.48 (homogeneity of the scale: 0 represents no correlation and 1 represents a perfect Guttmann scale), with the lowest value for item 9 and the highest value for item 4. Cronbach's alpha for the scale is 0.84 (test score reliability coefficient: 0 represents no interitem correlation 1 represents perfect interitem correlation).
After fitting the GR IRT model, test characteristics were examined. The test characteristic curve (Fig. 2) shows the expected overall observed sum-score of the FreBAQ-G at each given θ-value of the underlying trait (back specific self-perception), which is plotted on the x-axis. It can be seen that the test characteristic curve is an increasing, nonlinear function of the underlying trait. This was anticipated because subjects with a higher perceptual impairment (right side of the x-axis) will be expected to score higher on the FreBAQ-G sum score. The test characteristic curve shows that 95% of the people with NSCLBP will score between 1 and 21 on the classical FreBAQ-G sum-score. In people with NSCLBP, who have an average self-perception, the sum-score will be 7, whereas a person with a self-perception 1 SD worse than average will reach a value of 14, and if the self-perception is 1 SD better above average, the score will drop to 2. Keeping in mind that higher values are an expression of more impairment, the figure also shows that the questionnaire can better differentiate people with a higher degree of impaired back specific self-perception, whereas people with a lower degree of perceptual impairment, the questionnaire offers only 7 points for discrimination (between zero points and seven points).
Graph: Fig. 2 Test characteristic curve; y-axis: expected sum-score; x-axis = estimated Theta values, with a mean = 0 and a SD = 1, 95% of all patients with NSLBP have a Theta value between − 1.96 to 1.96 or between 0.984 to 20.7 in the FreBAQ sum-score
The test information function curve (Fig. 3) confirms that the FreBAQ-G is most informative for persons with a self-perception of the back worse than average (Theta between 0 and + 4).
Graph: Fig. 3 Test information function and standard error; Y-axis left side = test information; y-axis right side = standard error; x-axis = estimated Theta values
The graph also shows that for persons with a self-perception of the back worse than the average, the SE (standard error of measurement) is lower than for those with a better than average self-perception of the back. The overall reliability of the FreBAQ-G in this study is 0.84. The reliability for self-perception of the back better than average is 0.797 and 0.884 for worse than the average.
Table 3 gives an overview of the item characteristics for the FreBAQ-G. All discrimination values of the nine items are above the desired value of 1. The highest discrimination parameter showed item 4, the lowest item 9. Because item discrimination and item information are related concepts it can be expected that item 4 offers the highest information whereas item 9 offers the lowest information about individual back specific self-perception. Table 3 also gives the item difficulty parameters for each item and category. The interpretation of each of the four item difficulty parameters per item is that a person equal to it has a probability of 50% for responding in the pertinent category or higher. For example, looking at the estimate of 0.87 for item 1 ("My back feels as though it is not part of the rest of my body") category ≥2 means that a person with a back specific self-perception (Theta value) of that value (that is 0.87) has a probability of 50% to answer with category 0 or 1 versus response category 2 or higher (being 2, 3 or 4). Similarly, someone with a Theta value of − 0.07 has the same probability to answer 0 as to answer 1 or higher.
Table 3 Item characteristics FreBAQ-G
Nr Item Item dicrimination (a) Item difficulty (b) Info Rank ≥ 1 ≥ 2 ≥ 3 = 4 1 My back feels as though it is not part of the rest of my body 1.57 − 0.07 0.87 2.16 3.91 5 2 I need to focus all my attention on my back to make it move the way I want it to 1.39 −0.89 0.30 1.57 3.15 3 3 I feel as if my back sometimes moves involuntarily, without my control 1.85 −0.09 1.06 2.16 3.91 6 4 When performing everyday tasks, I don't know how much my back is moving 2.31 −0.26 0.66 1.44 2.80 9 5 When performing everyday tasks, I am not sure exactly what position my back is 2.25 −0.20 0.60 1.35 2.47 8 6 I can't perceive the exactly outline of my back 2.17 0.34 1.30 1.82 2.70 7 7 My back feels like it is enlarged (swollen) 1.41 0.85 1.66 2.74 – 4 8 My back feels like it has shrunk 1.30 1.35 2.38 3.66 – 2 9 My back feels lopsided (asymmetrical) 1.03 −1.26 −0.10 1.24 3.13 1
Item discrimination parameter (
Figure 4 shows the category characteristic curve of item 4 ("When performing everyday tasks, I don't know how much my back is moving"). It can be seen that only the first and last response categories are monotonically decreasing and increasing. Furthermore, the categories can be considered as ordinal measures, so if one moves from left (not impaired) to the right (highly impaired) on the x-axis the probability of the response to the question of item 4 changes from category 0 (never feels like that) to category 4 (always, or most of the time feels like that). The crossing points can be described as transitions points. For instance, a Theta value of − 0.17 indicates a person whose back self-perception is 0.17 SD better than the average. In addition, for a person with a Theta value > − 0.17 the probability of choosing category 1 (rarely feels like that) becomes for the first time higher than choosing the reference category 0 (never feels like that). It is important to notice that each category has an interval where the probability for this response category is highest and that the intervals between the response categories are reflecting the ordinal structure of the questionnaire.
Graph: Fig. 4 Category characteristic curve of item 4 showing the probability of highest response. y-axis: probability of response option; x-axis: estimated Theta values; Because of ordered responses the curves are arranged from category zero (K = 0, left side) to category 4 (K = 4, right side). To ease interpretation vertical lines at the crossing points of the response categories were added. A respondent with a Theta lower than − 0.17 is likely to respond category 0, a respondent with a Theta between − 0.17 to 0.7 is likely to respond category 1 and so on
Examination of all category characteristic curves showed that for item 6 category 2 and for item 7 category 1 did not have a clear interval in the latent trait (see Fig. 5). Furthermore, none of the participants in the current German sample had sufficiently disrupted self-perception to choose category 4 (always, or most of the time feels like that) for items 7 and 8. The response categories of all items are shifted to the right side of the x-axis (e.g. see Fig. 6), except the categories of item 9 who are more equally distributed (see also Table 3).
Graph: Fig. 5 Category characteristic curve of item 6 (a) and item 7 (b). The response categories of item 6 and 7 are not properly ordered (compare to Fig. 4). For item 7 there is no region on the Theta-scale where the probability of choosing category 1 is higher than for the other categories. For item 6 this can be observed for response category 2
Graph: Fig. 6 Calibrated mean item difficulty parameters for the German (y-axis) and the English (x-axis) language version. Scales on x- and y-axis represent item difficulty values and Theta values. All mean item difficulties are between 0 and 4 therefore the range of Theta values is only between 0 and 4. Line of identity with a slope of 1, dotted lines indicate the 95% CI boundaries
Only item 8 shows a small degree of differential item function (DIF) indicating that gender may influence the responses as for item 8 the item discrimination is lower in females (male 1.95; female 1.09), and item difficulty parameters are shifted to the right (thresholds: female: 1.48, 1.72, 4.16; male: 0.9, 1.63, and 2.5). However, these differences are not significant (p = 0.51). For all other items and subgroups we found no DIF in the German sample.
The calibrated mean item difficulties were plotted with the German items on the y-axis and the English items on the x-axis (Fig. 6). All difficulty parameters, except for item 7, lie within the 95% CI borders indicating equivalence of the item difficulty parameters of the English and German-language versions.
Item discrimination parameters (a) of both populations were plotted in a similar way with the line of identity and 95% CI (Fig. 7). Again, all items lies within the 95% CI borders, indicating equivalence of the English and German versions. Values higher than 1 are desirable and indicate better discrimination. Items 4, 5 and 6 have the highest discrimination parameter in both populations.
Graph: Fig. 7 Item discrimination parameters for the German (y-axis) and the English (x-axis) language versions. Scales on x- and y-axis represent discrimination values. Blue line: line of identity with a slope of 1, dotted lines represent 95% CI. Higher item discrimination values are indicating better discrimination and higher information. Attention: the y and x-axis are not representing Theta values
After accounting for the uniform differential item function (DIF) of Item 7 the graded response model (GRM) estimation of both groups showed a mean Theta of 0 with a variance of 1 for the English-language population (anchor) compared to a mean of − 0.1006 with a variance of 0.8348 for the German-language population. The test characteristic curve (Fig. 8) shows that individuals in the German population with a back specific self-perception below-average (Theta between 0 and + 4) will score one point lower on the FreBAQ than individuals with the same level of body perception in the English-language population. For individuals with above-average body perception (Theta 0 to − 4) there is no obvious difference in the expected sum-score.
Graph: Fig. 8 Test characteristic curve for the English and German-language population. Vertical and horizontal lines are displaying the expected sum-score for Theta values 0, 1, 2, 3 and 4
Self-perception of the back showed significant, moderate correlations with pain intensity, disability and fear avoidance beliefs (Table 4).
Table 4 Correlations between the FreBAQ-G and pain related parameters
Theta FreBAQ Pain intensity Pain disability FABQ (sum score) FABQ (activity) FABQ (work) 1 0.421 1 0.402 0.689 1 0.42 0.365 0.347 1 0.334 0.316 0.329 0.695 1 0.346 0.284 0.25 0.887 0.286 1
FABQ sum-score of the Fear avoidance beliefs questionnaire; all correlations are sig p < 0.001.
Multiple linear regression with the FreBAQ-G as the dependent variable showed that the best prediction model includes pain intensity, pain related disability and the sum-score of the FABQ (R-squared 0.27, p < 0.001). Eta square values indicate that the FABQ has the highest influence (0.092) and pain related disability the lowest (0.029). Other variables (e.g. demographic data) had no significant influence on the FreBAQ-G.
The primary aim of this study was evaluate psychometric properties of the FreBAQ-G using IRT in a large sample of patients with NSCLBP. Our results indicate that the FreBAQ-G is a suitable questionnaire to measure impaired back specific self-perception, and has comparable properties to the English-language version [[
The FreBAQ-G showed good internal consistency (Cronbachs alpha 0.84), a good overall reliability (r = 0.84) and weak to moderate scalability (Loevinger Hj between 0.34 and 0.48). The questionnaire demonstrated unidimensional properties with factor loadings between 0.57 and 0.80 and at least moderate correlations (r > 0.35) with pain intensity, pain related disability and FABQ total - and subscores.
The participant characteristics in German-language study were comparable to those in the English-language study [[
The average total FreBAQ-G score in our sample was slightly lower with an average of 8.0 (SD 6.0) compared to Wand et al. [[
The calculated reliability index of our study was r = 0.84 (range 0.79–0.88) and Cronbach's Alpha was 0.84. These values are comparable to those found by Wand et al. [[
In line with Wand et al.'s study, we found that the scale of the FreBAQ-G reflects an unidimensional construct. Compared to Wand et al. [[
The FreBAQ-G is suitable to differentiate between people with back specific self-perception between Theta − 1 and + 2 (− 4 stands for not impaired and + 4 stands for highly impaired) (see also Fig. 3). For people with a better back specific self-perception perception (Theta values below − 1), other items have to be developed. This view is supported when looking at the FreBAQ-G sum-score that ranges from 0 to 45. People with an average impairment of self-perception of the back had a median score of 8. This means that for patients with worse than-average self-perception the score ranges from 8 to 45 whereas for patients better than-average, the available score range is only from 0 to 8.
Furthermore, the classical sum-score of the FreBAQ-G should be used with care. The sum-score can only have integer values, and because of the non-linear relationship between the sum-score and the theta score, the assumption of equal distances between scores is violated. Therefore the sum-score is not interval scaled and even its ordinal scale can be questioned.
We used the graded response model (GRM) to compare the performance of both language versions. The English-language version showed very good model fit for the graded response model (GRM). All items showed comparable item discrimination parameters. In both populations items 4, 5 and 6 showed the highest discrimination. Therefore, in both language versions, the responses to these three items are offering the highest amount of information about the back specific self-perception of the respondent. Only in the English-language version Items 8 and 1 had discrimination parameters lower than 1. We found no non-uniform differential item function (DIF). However we found uniform differential item function (DIF) in item 7. This does not automatically indicate that translation of these items was not accurate. Differential item function (DIF) can also occur due to chance, different sample sizes, group differences in age, sex or disease characteristics, different administration modes and real differences between cultural and language settings [[
Some limitations of our study need to be discussed. First of all, the sample size is at the lower margin of sample sizes recommended for IRT studies. However, our results are sufficiently precise to assume that a larger sample size would not alter their magnitude or direction. Also, levels of pain intensity were relatively low in comparison to Wand et al's study [[
Our results indicate that the FreBAQ-G gives the most valid results for persons with NSCLBP and physical impairment above average. The item discrimination values of item 4, 5 and 6 show that these provide the most information in regards to impaired back specific self-perception.
For clinical interpretation of FreBAQ-G sum-scores, a common metric is very helpful, especially when the results of different measurement instruments have to be compared [[
Graph
T-scores have a range from 0 to 100 with a mean of 50 and a SD of 10. Higher values indicate more pronounced perceptual impairment of the back. T-scores are interval-scaled, easy to interpret and T-scores from different questionnaires can easily be compared. E.g. if a person with NSCLBP has a T-score of 65 we know that their body perception is 1.5 SDs worse than average, or if another person has a T-score of 40 the body perception is 1 SD better than average. Assessment of impaired self-perception could add information to the complex clinical picture of NSCLBP and thereby help clinicians to select targeted treatment options.
Future longitudinal studies should investigate sensitivity to change of the FreBAQ. It would add confidence to the validity of the questionnaire, if improvement in outcomes such as pain, function or health-related quality of life correspond to changes in self-perception of the back. Furthermore, the FreBAQ could be used as an outcome measure in controlled trials investigating interventions that target impaired self-perception of the lower back, such as tactile discrimination training or visual feedback training, results may help to explore how treatments work or why treatments do not work.
The FreBAQ-G is best suited for determining impaired back specific self-perception in patients with NSCLBP who have worse than average self-perception. Our results indicate cross-cultural equivalence that is important for the comparison of international study results. We found positive correlations of impaired back self-perception to pain intensity, disability and fear avoidance beliefs, in line with previous findings.
We hereby acknowledge the contribution of Dr. Martin Rabey who provided the FreBAQ validation data set of the original English version for cross-cultural comparison. Many thanks!
All authors discussed the results and commented on the manuscript. AS and TSK developed the study design and methods of data analysis. TSK, AS and KL were responsible for data collection and quality monitoring. BW and KE contributed to the interpretation of data. All authors were involved in drafting the article and revising it critically for important intellectual content and gave final approval of the version to be published.
Open Access funding enabled and organized by Projekt DEAL.
The datasets during and/or analysed during the current study are available from the corresponding author on reasonable request.
The study was approved by the Human Research Ethics Committee of the University of Applied sciences (HAWK) in Hildesheim, Germany. Ethical approval date was 18/03/2019. All participants provided written informed consent to participate.
Not applicable.
None declared.
Graph: Additional file 1: Appendix Supplementary statistical analysis
• df
- Degree of freedom
• FABQ
- Fear avoidance and Beliefs Questionnaire
- FreBAQ-G
- Fremantle back awareness questionnaire German version
• IRT
- Item Response Theory
• LBP
- Low back pain
• NSCLBP
- Non-specific chronic low back pain
• NRS
- Numerical rating scale
• PCA
- Principal Component Analysis
• RMSEA
- Root mean square error of approximation
• SD
- Standard Deviation
• SE
- Standard error
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
By Axel Schäfer; Benedict M. Wand; Kerstin Lüdtke; Katja Ehrenbrusthoff and Thomas Schöttker-Königer
Reported by Author; Author; Author; Author; Author