590
P. Enthoven et al.
Cronbach’s alpha (26) was used to measure the internal
consistency or internal reliability of the PEI. If 3 studies show
Cronbach’s alphas between 0.85 and 0.90 this indicates strong
evidence for good internal consistency (27).
The relationships between the PEI and other measures were
investigated with Spearman’s rank correlation (r Spearman ), using
the following coefficients: 0–0.25 none to little; 0.25–0.50 fair;
0.50–0.75 moderate to good; > 0.75 very good to excellent (28).
The SRC was estimated by a measure integrating anchor- and
distribution-based approaches. The SRC value is the optimal cut-
off point of the receiver operating characteristic curve (SRC ROC )
(29). The GPE, dichotomized into importantly changed (very
much or much improved) or not importantly changed (slightly
improved, unchanged, or slightly worsened), was used as the
external criterion (anchor). Six participants reported greater de-
terioration, and were excluded from further analyses, since this
number was too small to determine the SRC for deterioration. The
Spearman’s correlation (r Spearman ) between the GPE and PEI was
used to examine whether the anchor was adequate; a correlation
coefficient of at least 0.5 has been recommended (27).
The distribution of the PEI for participants who were im-
portantly improved or not (anchor) was described. The sensitivity
and specificity were calculated to determine the ROC cut-off point
for each PEI score. To construct the ROC curve, the combination
of sensitivity and 1-specificity was plotted for each PEI score.
The SRC ROC , defined as the optimal cut-off point, is found on the
ROC curve where the sum of the percentages of misclassified
participants is lowest (29). Furthermore, positive predictive value
(PPV) estimates the proportion of participants who actually had
a high PEI score from the total number of participants classified
as importantly improved. The negative predictive value (NPV)
estimates the proportion of participants that actually had a low
PEI score from the total number of participants that were not
improved. Values of PPV and NPV that are close 1.00 suggest a
higher probability of correctly classifying participants into im-
proved/not improved. Statistical analyses were conducted using
IBM SPSS, version 23. The level of significance was set at 0.05.
No imputation for missing values was performed.
RESULTS
Participant characteristics
A total of 738 participants answered the questionnaire
at the start of the treatment (not shown in Table II). A
total of 159 subjects did not answer the questionnaire
after treatment (WAD study n = 16, CR study n = 25,
MixCP study n = 118, of whom 30 did not complete
the treatment, and for 88 participants the reason for
non-completion was unknown).
A total of 579 participants answered the questionnaires
after treatment and 516 participants completed the Neck
Disability Index/Functional Rating Index (NDI/FRI), the
GPE item “Compared with before treatment, how would
you describe your problems now?”, and all items of the
PEI after treatment (Table II). Completers were less often
living alone than non-completers (n = 108 (20.9%) vs
n = 63 (28.9%), respectively, p = 0.020). Completers were
less often worried about their finances than non-comple-
ters (n = 217 (54.8%) vs n = 114 (65.9%), respectively,
p = 0.014). Otherwise there were no significant differences
between the groups in participant characteristics (Table
II). Completers had lower EuroQol visual analogue scale
(EQ-VAS) scores than non-completers (mean (SD) 47.6
(21.0) vs 54.9 (20.9), respectively, p = 0.014). There were
no significant differences between the groups regarding
self-reported NDI/FRI, EQ-5D index, Anxiety and
Depression (Hospital Anxiety and Depression Scale;
HADS), and current work ability.
For the participants who completed the NDI/FRI, the
GPE, and all items of the PEI after treatment (Table
II), the mean (SD) age was 45.1 (10.1) years, and 385
Table II. Baseline characteristics of the participants who completed the Neck Disability Index (NDI) or Functional Rating Index (FRI),
the Global Perceived Effect (GPE), and all items of the Patient Enablement Instrument (PEI) after treatment
All (n = 516)
n (%)
Age, years, mean (SD)
Sex, women
BMI
Overweight
Obese
Country of origin
Sweden
Other Scandinavian
Non-Scandinavian
Living alone
Education
Compulsory
High school
University/college
Other
Working status
(Self-) employed/student, part- or full time
Unemployed
Other
Sick leave (yes) n = 116, 109, 283
Worries about finances often/quite often
Expectations for treatment: full recovery/some improvement
Probability of working within 6 months: very large or large, n = – , 102, 163
WAD (n = 116)
n (%)
CR (n = 115)
n (%)
45.1 (10.1)
385 (74.6) 39.8 (11.2)
81 (69.8) 48.9 (7.4)
67 (58.3) 45.7 (9.8)
237 (83.2)
192 (38.6)
126 (25.4) 41 (36.0)
22 (19.3) 54 (47.8)
23 (20.4) 97 (35.9)
81 (30.0)
–
–
–
108 (20.9) 105 (91.3)
5 (4.3)
5 (4.3)
26 (22.4) –
–
–
24 (20.9) 204 (73.6)
4 (1.4)
69 (24.9)
58 (20.4)
–
–
–
– 66 (23.3)
162 (57.2)
55 (19.4)
–
–
–
–
–
379 (73.9)
86 (16.8)
48 (9.4)
239 (47.0)
217 (54.8)
356 (69.8)
208 (57.1)
7 (6.1)
61 (53.0)
41 (35.7)
6 (5.2)
108 (93.1)
5 (4.3)
3 (2.6)
13 (12.9)
36 (31.9)
83 (72.2)
–
93 (81.6)
7 (6.1)
14 (12.3)
63 (57.8)
–
109 (95.6)
91 (89.2)
Data are reported as numbers (percentages) unless stated otherwise.
WAD: whiplash-associated disorders; CR: cervical radiculopathy; MixCP: mixed chronic pain; SD: standard deviation; BMI: body mass index.
www.medicaljournals.se/jrm
MixCP (n = 285)
n (%)
178 (62.9)
74 (26.1)
31 (11.0)
169 (59.7)
181 (64.0)
164 (58.4)
117 (45.7)