Post on 25-Mar-2023
Psychometric characteristics of the Parkinson’s disease questionnaire
(PDQ-39)—Ecuadorian version
Pablo Martınez-Martına,*, Marcos Serrano-Duenasb,c, Veronica Vaca-Baqueroc
aNeuroepidemiology Unit, Department of Applied Epidemiology, National Centre for Epidemiology, Carlos III Institute of Public Health, Madrid, SpainbMovement Disorders Unit, Department of Neurology, Carlos Andrade Marın Hospital, Quito, Ecuador
cSchool of Medicine, Catholic Pontifical University of Ecuador, Quito, Ecuador
Received 16 December 2004; revised 11 January 2005; accepted 7 February 2005
Abstract
This study sought to analyse certain metric characteristics of the Ecuadorian version (EV) of the Parkinson’s Disease Questionnaire (PDQ-
39 EV). A cross-sectional study was conducted on 137 Parkinson’s disease (PD) patients attending a Movement Disorders Unit.
Neurologists’ assessments were based on Hoehn and Yahr (HY), Schwab and England and Unified Parkinson’s Disease Rating Scales.
Patients’ self-evaluations included the Hospital Anxiety and Depression Scale, the Parkinson’s Disease Quality of Life questionnaire (PDQL
EV), and the PDQ-39 EV.
Analyses for acceptability, internal consistency, precision, and construct validity (convergent and known-groups) were performed.
Distribution of scores was satisfactory. There was no evidence of floor or ceiling effects. Although the alpha coefficient exceeded 0.70 for
mobility, activities of daily living (ADL) and stigma, it was nevertheless low for bodily discomfort (0.48), communication (0.40), and social
support (0.33). Fourteen items yielded low correlation coefficients (!0.40) with their respective dimensions. Correlation of social support
and bodily discomfort with the Summary Index (SI) was modest (0.46 and 0.36, respectively). PDQ-39 EV SI convergent validity with the
PDQL EV SI was very high (rSZK0.91), and known-groups validity proved satisfactory.
Results agreed in part with those yielded by an international study, identifying specific flaws probably linked to socio-cultural influence.
q 2005 Elsevier Ltd. All rights reserved.
Keywords: Health-related quality of life; Parkinson’s disease; PDQ-39 Ecuadorian version; Psychometric analysis; Assessment
1. Introduction
Parkinson’s disease (PD) is a chronic, progressive,
disabling condition. It typically manifests as a complex
movement disorder, including rigidity, bradykinesia, tre-
mor, gait and balance disturbances. In addition, a constella-
tion of non-motor symptoms, such as orthostatism,
dysphagia, constipation and mental disorders (depression,
anxiety, dementia, psychosis), may also be present [1,2].
Increasing disability and decreasing quality of life are
linked to progression of the disease [3,4]. It is therefore
important that disease-centred clinical rating scales as well
1353-8020/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.parkreldis.2005.02.003
* Corresponding author. Address: Unidad de Neuroepidemiologıa,
Centro Nacional de Epidemiologıa, Instituto de Salud Carlos III, C/Sinesio
Delgado 6, 28029-Madrid, Spain.
E-mail address: pmartinez@isciii.es (P. Martınez-Martın).
as measures capable of assessing the impact of illness from
the patient’s point of view are both available. This approach
constitutes the conceptual framework for application of
health-related quality of life (HRQoL) measures [5–7].
In PD, HRQol research has identified health problems
that have usually been ignored or underestimated, as well as
relevant determinants of perceived health. Furthermore, the
use of HRQoL measures enables changes in health status
induced by therapeutic interventions or disease progress to
be evaluated [8–11].
HRQoL is a multidimensional construct including
physical, mental and environmental domains [12–14]. The
most important HRQoL dimensions in PD have been
determined in previous studies [15,16] and are embedded
in the above basic generic domains. It must be stressed,
however, that some of these (self-image/stigma, role and
social function, and sexual function) have never or rarely
been included in clinical evaluation.
The Parkinson’s disease Questionnaire (PDQ-39) is the
most frequently used HRQoL-specific PD measure [17,18],
Parkinsonism and Related Disorders 11 (2005) 297–304
www.elsevier.com/locate/parkreldis
P. Martınez-Martın et al. / Parkinsonism and Related Disorders 11 (2005) 297–304298
and has been adapted to different cultures and languages,
including Spanish as spoken in Spain [19–21]. Its psycho-
metric characteristics—reliability, convergent and discri-
minant validity, and responsiveness—are satisfactory, and
the questionnaire has been applied in clinical trials [10,22].
To ascertain whether HRQoL measures may be reliably
used outside the original context in which they were
developed, a cross-cultural adaptation and validation study
should be undertaken in the new target setting [23–25].
2. Methods
2.1. Objective
The present study sought to analyse certain relevant
metric characteristics of the Ecuadorian version of the PDQ-
39 (PDQ-39 EV): acceptability, internal consistency,
precision, convergent and known-groups validity.
2.2. Design
Cross-sectional, one-point-in-time evaluation.
2.3. Patients
A random study was conducted on patients who had
diagnosis of PD according to United Kingdom PD Society
Brain Bank criteria [26,27], were in stages 1 to 5 of the
Hoehn and Yahr classification (HY) [28], and were
regularly followed-up at the Movement Disorders Unit,
Neurology Department, Carlos Andrade Marın Hospital
(Quito, Ecuador).
Exclusion criteria were: (1) illiteracy; (2) neurological
impairment or disability due to any condition other than PD
(e.g., hemiplegia or blindness); (3) concomitant severe
disease or cognitive impairment which, according to clinical
judgment, could interfere or decrease the reliability of self-
assessment; and (4) lack of informed consent.
2.4. Assessment
After obtaining subjects’ informed consent, raters
recorded the following: demographic (age, gender) and
historical data (age at onset of PD, years in treatment); HY
stage; Schwab and England scale (SES) [29]; Unified
Parkinson’s Disease Rating Scale Sections 1 to 3 (UPDRS)
[30]; and Short Portable Mental Status Questionnaire
(SPMSQ) [31]. Patients completed the Hospital Anxiety
and Depression Scale (HADS) [32] and HRQoL measures
independently.
The UPDRS is a combined scale (mental status,
activities of daily living, motor examination, and
complications). The first three subscales are devoted to
assessing manifestations of PD, and items are uniformly
scored (ranging from 0Znormal, to 4Zsevere), while
part four—Complications— possesses a heterogeneous
scoring system [30,33]. Attached to the UPDRS are a
modified HY [28,30] and the Schwab and England Scale
[29], a measure of functional independence providing
continuous judgment ranked according to 11 options
(from 100%, completely independent, to 0%, completely
dependent and bedridden). Ratings are neurologist-based,
and all scores are obtained by interview and examination.
The UPDRS is currently used as the reference scale in
clinical practice and research. A recent overview of its
characteristics and application was carried out by an ad
hoc Movement Disorder Society Task Force [34].
Pfeiffer’s SPMSQ is a 10-item scale rated by interview,
and is purpose-designed to detect and estimate the severity
of cognitive impairment. It explores memory (short- and
long-term), orientation (time, place and self), and basic
calculations. The SPMSQ is hardly influenced by edu-
cational level and, as a result, proves quite useful and
reliable for screening general and elderly populations. Five
errors or more are indicative of moderate or severe
impairment [31].
The HADS is used as a self-administered screening tool
for mood disorders in non-psychiatric outpatients attending
hospital consulting rooms [32]. The HADS authors tried to
prevent the presence of somatic items susceptible to being
scored as mood problems (e.g., bradykinesia interpreted as
slowness linked to depression). The scale consists of 14
items (7 for assessment of anxiety and 7 for depression),
scoring in each case from 0 (no problem) to 3 (extreme
problem). ScoresR11 points for a subscale are indicative of
mood disturbance (anxiety or depression). The psycho-
metric properties of the HADS in PD patients have been
recently reviewed [35].
HRQoL evaluation was carried out using the Ecuadorian
versions of the Parkinson’s disease Quality of Life
Questionnaire (PDQL) and PDQ-39.
The PDQL [36,37] is a PD-specific quality-of-life
instrument. It contains 37 items grouped in four domains,
namely, parkinsonian symptoms (14 items), systemic
symptoms (7 items), emotional function (9 items), and
social function (7 items). The time scope is ‘the last three
months’ and possible scores for each item range from 1 (all
the time) to 5 (never). To score each domain, mean scores
for the items are calculated. A summary index (PDQL SI)
can be obtained from the mean scores for all items. The
higher the index, the better the HRQoL.
The PDQ-39 includes 39 items in 8 domains: mobility
(10 items); activities of daily living (ADL) (6 items);
emotional well-being (6 items); stigma (4 items); social
support (3 items); cognition (4 items); communication (3
items); and bodily pain (3 items) [17]. Questions refer to the
preceding month. Scores for each item range from 0 (no
problem) to 4 (continuous problem/unable to do it). Scoring
for each dimension is calculated by the sum of scores for
each item divided by the maximum possible score of the
dimension and is expressed as a percentage. The PDQ-39
P. Martınez-Martın et al. / Parkinsonism and Related Disorders 11 (2005) 297–304 299
Summary Index (PDQ-39 SI) is calculated by the sum of the
scores for the dimensions divided by the number of domains
[38]. The lower the index, the better the HRQoL.
The tasks of translation and back translation were
respectively and independently undertaken by two medical
experts fluent in English and two bilingual non-experts. This
resulted in a definitive Ecuadorian Spanish version, deemed
to be equivalent to the original questionnaire, being agreed
upon.
The HRQoL questionnaires and the HADS were
completed by the patients themselves, immediately fol-
lowed by supervision in order to prevent the occurrence of
mistakes such as missing or duplicate answers. In the case of
patients with fluctuations, measurements were applied to an
‘on’ situation. The entire evaluation was completed within
an average of 3 days (range: 1–15).
This study was formally approved by Teaching and
Research Management at the Carlos Andrade Marın
Hospital (Ecuadorian Institute of Social Security).
2.5. Analysis
The following psychometric aspects of the PDQ-39 EV
were evaluated: acceptability, internal consistency and
construct-related validity.
Acceptability is the property whereby score distributions
adequately represent the true distribution of health status in
the sample [39]. Central tendency measures (mean, standard
deviation, range and median), floor and ceiling effects and
skewness for PDQ-39 EV dimensions and SI were
calculated in order to provide information about the
appropriateness of the questionnaire’s variability for its
intended use [40]. It was hypothesized that PDQ-39 EV
acceptability would be considered appropriate if the
following requirements were fulfilled: (1) proximity of
observed to possible score range, and mean score close
to mid-point (median); (2) low floor and ceiling effects
(!15%) [41]; and (3) limited skewness in distribution of
scores (from K1 to C1) [39,42].
Internal consistency refers to the precision of the scale,
based on the homogeneity (intercorrelation) of the scale’s
items at one point in time [40]. It was determined by
means of Cronbach’s alpha coefficient, an estimate of
reliability based on all possible split-half correlations. An
alpha value R0.70 was considered the minimum accep-
table value for group comparisons [40,43]. For individua-
lized use, standard error of measurement ½SEMZSD!
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
ð1K reliability coefficientÞp
�, which simultaneously
reflects reliability and degree of precision of measurement,
is recommended [40,41,44]. The SEM is equal to 0 when
reliability is absolute, but reliability coefficients (alpha for
a single observation) lower than 0.90 furnish intervals that
are too wide for individual use [40]. In such a case,
calculation of 1.96!SEM (a more conservative approach
which represents the 95% confidence interval, 95% CI)
may prove useful for estimating the relationship between
observed and ‘true’ values [41,44–46].
Validity is the degree to which an instrument measures
what it purports to measure. Within this concept, construct-
related validity may be defined in pragmatic terms as ‘the
degree to which an instrument measures the construct that it
was designed to measure’ [43]. PDQ-39 EV convergent
validity was explored: (a) internally, by evaluation of item-
dimension total, inter-dimension, and dimension-PDQ-39
EV Summary Index (PDQ-39 EV SI) correlation (corrected
for overlap when appropriate); and (b) with respect to other
PD measures. Spearman rank coefficient was used because
the data did not show a normal distribution. Known-groups
validity vis-a-vis severity levels (HY Staging: 1–2.5ZMild;
3ZModerate; 4–5ZSevere) [47] was determined by the
Kruskal–Wallis test.
The coefficient of variation (SD/mean) has the advantage
of being independent of the units of measurement. It was
calculated as an index of discriminating power to be
compared between scales. Higher values reflect a greater
ability for discrimination [47,48].
All data-analyses were performed using the STATAw
computer software program (version 8.2) [49].
3. Results
The study covered 137 PD patients, 93 of whom (67.9%)
were males; the age (meanGstandard deviation) of the
sample was 69.4G10.1 years (range: 44–92). Disease
duration was 5.9G2.6 years (range: 1–15), and levodopa-
treatment duration 4.7G2.5 years. All patients were
receiving levodopa (daily doseZ780.3G251.2 mg).
Additional bromocriptine treatment (the only dopamine
agonist provided by the Social Security in Ecuador) was
administered to 59 patients (daily doseZ10.8G4.9 mg).
Distribution of patients according to HY staging was as
follows: stage 1, 8.0%; stage 1.5, 19.7%; stage 2, 20.4%;
stage 2.5, 26.3%; stage 3, 18.25%; stages 4 and 5, 7.3%.
Descriptive characteristics of the sample are shown in
Table 1.
Observed scores covered the entire possible score range
(0–100) in three dimensions (mobility, stigma and bodily
discomfort) and were very close to this range in another two
dimensions (ADL and emotional well-being). In summary,
five out of the 16 extreme scores were truncated. Gaps of
more than 10% between observed and maximum scores
were observed for the three remaining dimensions (Table 1).
Differences between the mean and median for PDQ-39
EV dimensions were small, ranging from 0.4 (emotional
well-being) to 6.6 (ADL). For the PDQ-39 EV SI, this
difference was 1.07 (meanZ41.48; medianZ40.1).
For PDQ-39 EV dimensions, the floor effect ranged from
0.7% (mobility) to 13.14% (social support), and the ceiling
effect ranged from 0.7% (mobility, emotional well-being,
social support, communication, and bodily discomfort) to
Table 1
Descriptive characteristics of the sample
Mean SD Min. Max.
Age at study 69.4 10.1 44 92
Duration of disease 5.9 2.6 1 15
SPMSQ 1.4 1.6 0 6
Schwab and England Scale 70.4 19.4 10 100
HADS
Anxiety 8.7 3.9 0 19
Depression 10.4 4.3 1 18
UPDRS
Mental status 5.4 2.4 2 13
Activities of daily living 19.7 10 4 47
Motor examination 40.4 14.9 15 93
PDQ-39-EV
Mobility 48.5 23.9 0 100
Activities of daily living 39.9 26.3 0 91.6
Emotional well-being 49.6 16.8 7.8 100
Stigma 47.2 29.1 0 100
Social support 30.7 21.5 0 83.3
Cognitions 36.2 20.6 0 87.5
Communication 35.7 19.3 0 83.3
Bodily discomfort 43.8 22.7 0 100
Summary index 41.5 15.5 8.8 81.7
SD, standard deviation; Min., minimum; Max., maximum; SPMSQ, short
portable mental status questionnaire; HADS, hospital anxiety and
depression scale; UPDRS, unified Parkinson’s disease rating scale; PDQ-
39 EV, Parkinson’s disease questionnaire-39 Ecuadorian Version.
Table 3
PDQ-39 Ecuadorian Version: Item-dimension total correlation for items
with coefficients below the 0.40 criterion
Domain Item rS
P. Martınez-Martın et al. / Parkinsonism and Related Disorders 11 (2005) 297–304300
3.65% (stigma). For the summary index, both floor and
ceiling effects were 0.7%.
Whereas skewness ranged from 0.04 for ADL to 0.66 for
emotional well-being in the PDQ-39 EV, it was 0.61 in the
PDQ-39 EV SI.
The alpha values for the respective PDQ-39 EV
dimensions are displayed in Table 2. Mobility, ADL and
stigma registered coefficients higher than 0.70. The lowest
value (0.33) was obtained for social support and the highest
(0.93) for ADL.
SEM values for each dimension ranged from 6.76 for
mobility to 17.61 for social support (Table 3). As only two
Table 2
Internal consistency of the PDQ-39-Ecuadorian Version
PDQ-39 Dimensions Cronbach’s alpha Standard error of
measurement
EV UKa Spainb SEM 1.96!
SEMc
Mobility 0.92 0.94 0.93 6.76 13.25
Activities of daily living 0.93 0.89 0.91 6.96 13.64
Emotional well-being 0.60 0.83 0.83 10.62 20.81
Stigma 0.78 0.80 0.72 13.67 26.80
Social support 0.33 0.69 0.50 17.61 34.51
Cognition 0.64 0.70 0.70 12.34 24.18
Communication 0.40 0.79 0.73 14.95 29.30
Bodily discomfort 0.48 0.75 0.56 16.37 32.08
a Peto et al. [17]—(PDQ-39 Original Version).b Jenkinson et al. [19]—(PDQ-39 Spanish Version).c For comparison with Fitzpatrick et al. [46].
PDQ-39 EV domains registered reliability coefficients
R0.90, SEM 95% CI were calculated and are thus displayed
in Table 2.
Insofar as internal consistency is concerned, approxi-
mately two thirds of item-dimension correlation coefficients
attained values higher than the criterion of 0.40 [19,43,50].
Mobility and ADL accounted for the highest coefficients
(rSZ0.57–0.84), with the lowest being recorded for social
support and communication (Table 3).
In order to determine the consistency of each dimension
as a component of the PDQ-39 EV, both inter-dimension
and dimension-PDQ-39 EV SI correlation were determined.
The closest association between dimensions (rSZ0.70) was
registered for mobility and ADL, and the lowest (rSZ0.14)
for emotional well-being and bodily discomfort. While the
ADL and communication domains evinced a close associ-
ation with the PDQ-39 EV SI (rSO0.70), bodily discomfort
(rSZ0.36) and social support (rSZ0.46) demonstrated a
weaker association (Table 4).
Table 5 lists the correlation coefficients between PDQ-39
EV and the other PD measures applied in this study.
Mobility, ADL and the PDQ-39 EV SI displayed the highest
values. In contrast, stigma, social support and bodily
discomfort showed a loose association with these scales.
The PDQ-39 EV SI proved to be significantly correlated
with the PDQL EV SI (rSZK0.91; p!0.0001).
Mean PDQ-39 EV dimension scores and SI for each HY
stage are shown in Table 6. Known-groups validity for
HY-based severity levels was not satisfactory for emotional
well-being (Kruskal–Wallis test, pZ0.05) and proved
barely significant for bodily discomfort (pZ0.03).
The coefficient of variation was 0.37 for the PDQ-39 EV
SI and 0.21 for the PDQL EV SI.
Emotional well-being
18. Felt isolated and lonely 0.29
19. Felt weepy or tearful 0.30
21. Felt anxious 0.35
22. Felt worried about the future 0.18*
Social support
27. Problems with close relationships 0.31
28. Support from spouse or partner 0.22
29. Support from friends or family 0.23
Cognition
32. Felt your memory was bad 0.24
Communication
34. Difficulty with speech 0.35
35. Unable to communicate properly 0.21
36. Felt ignored by people 0.20
Bodily discomfort
37. Muscle cramps or spasms 0.36
38. Aches and pains 0.33
39. Unpleasant hot or cold 0.20
rS, Spearman rank correlation coefficients. *pZ0.03.
Table 4
Correlation between dimensions and dimension-summary index of the PDQ-39 Ecuadorian version
Mob ADL E w-b Stigma SocSup Cognit Com Body D SI
Mob 0.66
ADL 0.70 0.74
E w-b 0.38 0.44 0.56
Stigma 0.32 0.56 0.47 0.52
Soc Sup 0.31 0.39 0.25 0.26 0.46
Cognit 0.50 0.51 0.51 0.30 0.37 0.57
Com 0.50 0.60 0.40 0.53 0.54 0.45 0.72
Body D 0.42 0.28 0.14 0.23 0.20 0.23 0.30 0.36
Spearman rank correlation coefficients. rSR0.27, p!0.001 (Bonferroni correction) Mob, mobility; ADL, activities of daily living; E w-b, emotional well-
being; SocSup, social support; Cognit, cognition; Com, communication; Body D, bodily discomfort; SI, PDQ-39 summary index.
P. Martınez-Martın et al. / Parkinsonism and Related Disorders 11 (2005) 297–304 301
4. Discussion
Our study sample comprised patients representing a wide
range of ages (44–92 years) and PD duration (1–15 years).
All degrees of severity (HY stages 1–5) were represented,
though few patients were at the extremes of staging.
Insofar as acceptability is concerned, distribution of the
PDQ-39 EV scores was satisfactory, covering a wide range
of possible scores, both for dimensions and summary index
(Table 1). Mean and median were very close, with the
difference never reaching 10% of the possible maximum
score. There was no floor or ceiling effect higher than the
15% criterion [41], and skewness kept within the established
range of values, between K1 and C1 [39,42]. Acceptability
of the PDQ-39 EV was thus satisfactory.
In an international cross-sectional study covering six
countries (Global Parkinson’s disease Survey, GPDS) [51]
and 1020 patients (902 fully computable), local versions of
the PDQ-39 were applied. The ensuing data enabled
Jenkinson et al. [19] to evaluate the metric characteristics
of these cross-cultural adapted questionnaires for use in
the United States, Canada, Italy, Spain and Japan. It is
interesting to compare the results of this study against ours.
Although all stages of disease were represented in the
GPDS, the proportion of patients in stages 1 and 1.5 was
lower and that of patients in stage 4 and 5 higher than in
the Ecuadorian study.
Table 5
Convergent validity of the PDQ-39 Ecuadorian version with other Parkinson’s di
PDQ-39 HyY SES UPDRS I U
Mobility 0.74 K0.75 0.51 0
ADL 0.67 K0.75 0.54 0
E w-b 0.32 K0.30 0.25 0
Stigma 0.19 K0.23 0.24 0
SocSup 0.30 K0.24 0.36 0
Cognition 0.49 K0.47 0.42 0
Com 0.37 K0.34 0.32 0
Bodily D 0.22 K0.16 0.09 0
Summary index 0.60 K0.60 0.50 0
Spearman rank correlation coefficients. After Bonferroni adjustment, significant r
Floor effect in the GPDS surpassed the 15% criterion for
stigma, social support and communication in at least four of
the above five countries. This is a relevant difference vis-a-
vis our study, particularly in view of the fact that severity
was higher in the international study. Indeed, the PDQ-39 SI
was significantly higher in Ecuador (41.48G15.51) than in
the international combined study (32.2G19.7) (unpaired
t-test, p!0.0001), indicating worse HRQoL in Ecuadorian
patients despite lesser PD severity. In the GPDS, however,
there was no evidence of a ceiling effect for any dimension
or country, a finding in line with our results.
In the PDQ-39 EV, mobility, ADL and stigma attained
alpha values higher than the lower limit (0.70) considered
satisfactory for groups [40,43] (Table 2). Three other
dimensions (social support, communication and bodily
discomfort) obtained alpha coefficients lower than 0.50. In
the GPDS, social support and bodily discomfort registered
similar results, not only in country like Spain—a country
that is presumably linguistically and culturally closer to
Ecuador—but also in countries such as Japan or Italy. The
low alpha value for communication (0.40) observed in
Ecuador is unique when compared to the other countries.
The dimensions of social support, communication and
bodily discomfort contain only three items per dimension. A
low alpha coefficient for these dimensions could be
attributable to the direct relationship between alpha value
and length of scale [43]. Nonetheless, this potential
sease measures
PDRS II UPDRS III HADS-A HADS-D
.74 0.56 0.32 0.52
.81 0.56 0.37 0.51
.37 0.17 0.52 0.52
.37 0.20 0.38 0.57
.21 0.24 0.24 0.42
.51 0.35 0.46 0.40
.47 0.28 0.38 0.48
.29 0.09 0.19 0.10
.71 0.46 0.53 0.67
SR0.28 (p!0.0007).
Table 6
PDQ-39 scores broken down by disease stage
PDQ-39 Hoehn and Yahr stages Known-groupsa
1 1.5 2 2.5 3 4 and 5 p*
Mobility 28.18 (16.54) 27.13 (17.83) 36.16 (19.60) 57.63 (12.53) 70.80 (11.19) 75.25 (21.84) 0.0001
ADL 22.34 (18.93) 23.76 (17.32) 19.64 (14.33) 46.41 (19.36) 72.33 (16.79) 55.83 (23.91) 0.0001
Emotional well-being 37.87 (18.67) 43.98 (18.32) 45.38 (11.58) 55.32 (16.83) 57.00 (12.99) 50.41 (18.78) 0.05
Stigma 35.79 (26.82) 42.59 (29.87) 39.50 (29.41) 51.73 (32.09) 65.00 (16.73) 33.12 (23.02) 0.001
Social support 34.09 (17.65) 21.29 (21.09) 22.32 (17.86) 31.01 (22.23) 49.33 (11.76) 28.33 (25.51) 0.0001
Cognition 27.84 (15.14) 23.14 (16.05) 23.21 (13.37) 46.52 (20.50) 55.00 (15.41) 33.75 (7.90) 0.0001
Communication 39.39 (21.11) 23.76 (19.57) 28.87 (15.62) 35.41 (16.47) 55.00 (9.31) 35.83 (19.66) 0.0001
Bodily discomfort 46.21 (27.97) 31.17 (19.14) 37.20 (21.09) 50.46 (18.03) 56.66 (26.13) 38.33 (12.54) 0.03
Summary index 33.96 (10.62) 29.60 (12.73) 31.53 (9.54) 46.81 (11.56) 60.14 (9.62) 43.86 (10.87) 0.0001
Mean and (standard deviation). *Kruskal–Wallis test.a Known-groups based on the Hoehn and Yahr Staging (HY): Mild, HYZ1.0–2.5; Moderate, HYZ3.0; Severe, HYZ4–5.
P. Martınez-Martın et al. / Parkinsonism and Related Disorders 11 (2005) 297–304302
relationship has materialized in no socio-cultural context
other than the dimension of social support, an almost
constant finding that challenges the consistency of this latter
dimension and bars its use as a primary end point for clinical
research and such applications as clinical trials [19].
The SEM of the PDQ-39 EV domains ranged from 6.76
(mobility; alphaZ0.92) to 17.61 (social support; alphaZ0.33). In the present study, only two dimensions (mobility and
ADL) yielded alpha coefficient higher than 0.90. Accord-
ingly, SEM 95% CI were calculated for each dimension [41]
(Table 2). Recently, Fitzpatrick et al. [46] have argued in
favor of one-SEM criterion to represent the minimum change
score to be statistically meaningful for each dimension of the
PDQ-39. SEM is considered a measure of internal consist-
ency, required for individualized application [40,41,44,45].
The best reliability (1.0) would produce a SEMZ0.
Obviously, the lower the reliability, the greater the SEM,
the less the precision of the scale, and the wider the 95% CI.
As can be deduced from these results, the PDQ-39 EV
suffers from some relevant shortcomings insofar as internal
consistency and precision is concerned.
Except for mobility and ADL, the other dimensions
yielded a variable number of item-dimension correlation
coefficients below the established criterion of 0.40 [19,43,
50] (Table 3). As in the GPDS, all items belonging to the
PDQ-39 EV dimensions of social support and bodily
discomfort were included in this category, as were most
items of emotional well-being, a finding not present in the
GPDS. A multitrait-scaling analysis (not shown) failed to
demonstrate higher correlations between the emotional
well-being items and other scales. In order to complete the
information on this aspect, an inter-dimension and corrected
dimension-summary index correlation analysis was con-
ducted. Social support and bodily discomfort displayed a
weak association with the other dimensions and the lowest
correlation with the summary index (Table 4), indicative of
these domains having a loose convergent validity.
The PDQ-39 EV’s convergent validity with measures
usually applied for evaluation of PD (HY, SES, UPDRS,
HADS) proved satisfactory for mobility and ADL, modest
for cognition and communication, and poor for the
remaining domains. The summary index yielded significant
correlation coefficients (rSZ0.46–0.71) with all the applied
scales (Table 5). As a whole, these findings are in agreement
with previous studies [20,35,52–54] and confirm a satisfac-
tory association between certain PDQ-39 dimensions and
summary index on the one hand, and standard clinical scales
for PD and self-perceived mood status on the other.
HRQoL measurement reflects patients’ subjective point
of view, and this perception may only agree in part with the
clinical assessment [7,55]. Moreover, the objectives of
clinical ratings and HRQoL measures are different. There-
fore, a weak statistical association between these kinds of
instruments is not equivalent to uselessness or lack of
significance. Nevertheless, convergent validity with a scale
designed for the same purpose must necessarily be strong.
The correlation between the PDQ-39 EV and PDQL EV
summary indices (rSZK0.91) confirmed this psychometric
property adequately. A detailed analysis of the relationships
between both HRQoL measures is beyond the scope of this
article and will be reported on in the future.
Finally, Table 6 shows PDQ-39 EV mean scores broken
down by disease stage. Some dimensions (e.g. social
support, communication or bodily discomfort) register
lower scores in stages 1.5 and 2 than in stage 1. This
fluctuation has not been observed in previous studies [56]
using the same UPDRS-modified HY, and is perhaps linked
to an idiosyncratic phenomenon of patients adapting to
impairment and disability [55,57]. Except for bodily
discomfort, the PDQ-39 EV did, however, prove to be a
valid discriminative measure for distinguishing between
groups of patients with different degrees of severity. The
PDQ-39 EV SI coefficient of variation (0.37) was higher than
the corresponding index of the PDQL EV (0.21), indicating a
potentially better discriminative capacity [47]. Higher
values for the PDQ-39 SI coefficient of variation (0.58–
0.61) may be obtained from previous studies [22,38,51].
This study’s main limitations are due to the sample size
and composition, with a relatively low number of patients at
either extreme of the severity classification.
P. Martınez-Martın et al. / Parkinsonism and Related Disorders 11 (2005) 297–304 303
To sum up, significant shortcomings of internal consist-
ency have been identified in the PDQ-39 EV, but
acceptability, convergent validity with other measures,
and known-groups validity have all proved satisfactory.
These findings advise against the use of this PDQ-39 EV
while waiting for amendments or new studies that
substantiate the reliability of the instrument in this social
context. Some specific findings revealed by this adaptation
may be linked to socio-cultural aspects that can
influence the way in which certain HRQoL measures work
[19,23,25,58]. Even aspects that are more external and
objective than the concept of quality of life may be tinged
by this type of influence [59].
Acknowledgements
R. Pastor-Barriuso, M.Sc., National Centre for Epidemiol-
ogy, provided statistical help. Logistic support for analysis of
data [computer and bibliography) was supplied by AEINN.
References
[1] Shulman LM, Teback RL, Bean J, Weiner WJ. Comorbidity of the
nonmotor symptoms of Parkinson’s disease. Mov Disord 2001;16:
507–10.
[2] Shulman LM, Teback RL, Rabinstein AA, Weiner WJ. Non-
recognition of depression and other non-motor symptoms in
Parkinson’s disease. Parkinsonism Relat Disord 2002;8:193–7.
[3] Schrag A, Jahansahi M, Quinn NP. What contributes to quality of life
in patients with Parkinson’s disease? J Neurol Neurosurg Psychiatry
2000;69:308–12.
[4] Schrag A, Selai C, Jahanshahi M, Quinn NP. How does Parkinson’s
disease affect quality of life? A comparison with quality of life in
general population Mov Disord 2000;15:1112–8.
[5] Guyatt G, Feeny D, Patrick DL. Measuring health-related quality of
life. Ann Intern Med 1993;118:622–9.
[6] Schipper H, Clinch JJ, Olweny CLM. Quality of life studies:
definitions and conceptual issues. In: Spilker B, editor. Quality of
life and pharmacoeconomics in clinical trials. 2nd ed. Philadelphia:
Lippincot-Raven Publishers; 1996. p. 11–23.
[7] Martinez-Martin P. An introduction to the concept of ‘quality of life in
Parkinson’s disease’. J Neurol 1998;245(Suppl 1):2–6.
[8] Peto V, Fitzpatrick R, Jenkinson C. Self-reported health status and
access to health services in a community sample with Parkinson’s
disease. Disabil Rehabil 1997;19:97–103.
[9] Karlsen KH, Larsen JP, Tandberg E, Maeland JG. Influence of clinical
and demograhic variables on quality of life in patients with
Parkinson’s disease. J Neurol Neurosurg Psychiatry 1999;66:431–5.
[10] Martınez Martın P. Health-related quality of life in Parkinson’s
disease: outcomes of the therapeutic interventions. Expert Rev
Pharmacoeconomics Outcomes Res 2001;1:99–108.
[11] Rubenstein LM, DeLeo A, Chrischilles EA. Economic and health-
related quality of life considerations of new therapies in Parkinson’s
disease. Pharmacoeconomics 2001;19:729–52.
[12] Berzon R, Hays RD, Shumaker SA. International use, application and
performance of health-related quality of life instruments. Qual Life
Res 1993;2:367–8.
[13] Spilker B. Introduction. In: Spilker B, editor. Quality of life and
pharmacoeconomics in clinical trials. 2nd ed. Philadelphia: Lippincot-
Raven Publishers; 1996. p. 1–10.
[14] Fitzpatrick R, Davey C, Buxton M, Jones D. Evaluating patient-based
outcomes measures for use in clinical trials. Health Technol Assess
1998;2:1–74.
[15] Damiano AM, Snyder C, Strausser B, Willian MK. A review of
health-related quality-of-life concepts and measures for Parkinson’s
disease. Qual Life Res 1999;8:235–43.
[16] Fitzpatrick R, Alonso J. Quality of life in health care: concepts and
components. In: Martinez-Martin P, Koller WC, editors. Quality of
life in Parkinson’s disease. Barcelona: Masson, SA; 1999. p. 1–15.
[17] Peto V, Jenkinson C, Fitzpatrick R, Greenhall R. The development
and validation of a short measure of functioning and well being for
individuals with Parkinson’s disease. Qual Life Res 1995;4:241–8.
[18] Marinus J, Ramaker C, van Hilten JJ, Stiggelbout AM. Health-related
quality of life in Parkinson’s disease: a systematic review of disease-
specific instruments. J Neurol Neurosurg Psychiatry 2002;72:241–8.
[19] Jenkinson C, Fitzpatrick R, Norquist J, Findley L, Hughes K. Cross-
cultural evaluation of the Parkinson’s Disease Questionnaire: tests of
data quality, score reliability, response rate, and scaling assumptions
in the United States, Canada, Japan, Italy, and Spain. J Clin Epidemiol
2003;56:843–7.
[20] Martınez Martın P, Frades Payo B. The ‘Grupo Centro’ for Study of
Movement Disorders. Quality of life in Parkinson’s disease.
Validation study of the PDQ-39 Spanish version. J Neurol 1998;
245(Suppl 1):34–8.
[21] Martinez-Martin P, Frades B, Jimenez Jimenez FJ, Pondal M, Lopez
Lozano JJ, Vela L, Vazquez A, del Val Fernandez J, Divi M,
Fabregat N, Tuldra A, Crespo Maraver MC, Molinero LM. The PDQ-
39 Spanish Version: reliability and correlation with the Short-Form
Health Survey (SF-36). Neurologıa 1999;14:159–63.
[22] Martinez-Martin P. Calidad de vida relacionada con la salud en la
enfermedad de Parkinson. Barcelona: Ars Medica; 2002.
[23] Cella DF, Lloyd SR, Wright BD. Cross-cultural instrument equating:
current research and future directions. In: Spilker B, editor. Quality of
life and pharmacoeconomics in clinical trials. 2nd ed. Philadelphia:
Lippincot-Raven publishers; 1996. p. 707–15.
[24] Anderson RT, McFarlane M, Naughton MJ, Shumaker SA. Concep-
tual issues and considerations in cross-cultural validation of generic
health-related quality of life instruments. In: Spilker B, editor. Quality
of life and pharmacoeconomics in clinical trials. 2nd ed. Philadelphia:
Lippincot-Raven Publishers; 1996. p. 605–12.
[25] McKenna SP, Whalley D. Coping with language barriers. Good Clin
Pract J 1997;4:14–17.
[26] Gibb WRG, Lees AJ. The relevance of the Lewy body to the
pathogenesis of idiopathic Parkinson’s disease. J Neurol Neurosurg
Psychiatry 1988;51:745–52.
[27] Hughes AJ, Daniel SE, Kilford L, Lees AJ. Accuracy of clinical
diagnosis of idiopathic Parkinson’s disease—a clinico-pathological
study of 100 cases. J Neurol Neurosurg Psychiatry 1992;55:181–4.
[28] Hoehn MM, Yahr MD. Parkinsonism: onset, progression, and
mortality. Neurology 1967;17:427–42.
[29] Schwab RS, England AC. Projection technique for evaluating surgery
in Parkinson’s disease. In: Gillingham FJ, Donaldson IML, editors.
Third Symposium on Parkinson’s disease. Edinburgh: E and S
Livingstone; 1969. p. 152–7.
[30] Fahn S, Elton RL, Members of The UPDRS Development Committee.
In: Fahn S, Marsden CD, Goldstein M, Calne DB, editors. Unified
Parkinson’s Disease Rating Scale. Recent development in Parkinson’s
disease, Vol. 2. New Jersey: McMillan; 1987. p. 153–63.
[31] Pfeiffer E. A short portable mental status questionnaire for the
assessment of organic brain deficit in elderly patients. J Am Geriatr
Soc 1975;23:433–41.
[32] Zigmond AS, Snaith RP. The hospital anxiety and depression scale.
Acta Psychiatr Scand 1983;67:361–70.
[33] Martinez-Martin P, Gil-Nagel A, Morlan Gracia L, Balseiro Gomez J,
Martınez Sarries J, Bermejo F, The Cooperative Multicentric Group.
Unified Parkinson’s Disease Rating Scale characteristics and
structure. Mov Disorders 1994;9:76–83.
P. Martınez-Martın et al. / Parkinsonism and Related Disorders 11 (2005) 297–304304
[34] Movement Disorder Society Task Force on Rating Scales for
Parkinson’s Disease. The Unified Parkinson’s Disease Rating Scale
(UPDRS): status and recommendations. Mov Disord 2003;18:738–50.
[35] Marinus J, Leentjens AFG, Visser M, Stiggelbout AM, van Hilten JJ.
Evaluation of the hospital anxiety and depression scale in patients
with Parkinson’s disease. Clin Neuropharmacol 2002;25:318–24.
[36] de Boer AGEM, Wijker W, Speelman JD, de Haes JCJM. Quality of
life in patients with Parkinson’s disease: development of a
questionnaire. J Neurol Neurosurg Psychiatry 1996;61:70–4.
[37] Serrano-Duenas M, Martinez-Martin P, Vaca Baquero V. Validation
and Cross-cultural adjustment of PDQL–Questionnaire, Spanish
version (Ecuador) (PDQL–EV). Parkinsonism Relat Disordrrs 2004;
10:433–7.
[38] Jenkinson C, Fitzpatrick R, Peto V, Greenhall R, Hyman N. The
Parkinson’s Disease Questionnaire (PDQ-39): development and
validation of a Parkinson’s disease summary index score. Age Ageing
1997;26:353–7.
[39] Riazi A, Hobart JC, Lamping DL, Fitzpatrick R, Thompson AJ.
Evidence-based measurement in multiple sclerosis: the psychometric
properties of the physical and psychological dimensions of three
quality-of-life rating scales. Mult Scler 2003;9:411–9.
[40] Scientific Advisory Committee of the Medical Outcomes Trust.
Assessing health status and quality-of-life instruments: attributes and
review criteria. Qual Life Res 2002;11:193–205.
[41] McHorney C, Tarlov A. Individual-patient monitoring in clinical
practice: are available health status surveys adequate? Qual Life Res
1995;4:293–307.
[42] Holmes WC, Bix B, Shea JA. SF-20 score and item distributions in a
human immunodeficiency virus-seropositive sample. Med Care 1996;
34:562–9.
[43] Fayers PM, Machin D. Quality of life. Assessment, analysis and
interpretation. Chichester: Wiley; 2000.
[44] Wyrwich KW, Nienaber MA, Tierney WM, Wolinsky FD. Linking
clinical relevance and statistical significance in evaluating intraindi-
vidual changes in health-related quality of life. Med Care 1999;37:
469–78.
[45] Nunnally JC, Bernstein IH. Psychometric theory. 3rd ed. New York:
McGraw-Hill, Inc; 1994 pp. 258–260.
[46] Fitzpatrick R, Norquist JM, Jenkinson C. Distribution-based criteria
for change in health-related quality of life in Parkinson’s disease.
J Clin Epidemiol 2004;57:40–4.
[47] Marinus J, Visser M, Verwey NA, Verhey FRJ, Middelkoop HAM,
Stiggelbout AM, van Hilten JJ. Assessment of cognition in
Parkinson’s disease. Neurology 2003;61:1222–8.
[48] Bland M. An introduction to medical statistics. 3rd ed. Oxford:
Oxford University Press; 2000 pp. 270–271.
[49] StataCorp. Stata Statistical Software: Release 8.0. College Station,
TX: Stata Corporation; 2003.
[50] Nanda U, McLendon PM, Andresen EA, Armbrecht E. The SIP68: an
abbreviated sickness impact profile for disability outcomes research.
Qual Life Res 2003;12:583–95.
[51] The Global Parkinson’s Disease Survey Steering Committee. Factors
impacting quality of life in Parkinson’s disease: results from an
international survey. Mov Disord 2002;17:60–7.
[52] Jenkinson C, Peto V, Fitzpatrick R, Greenhall R, Hyman N. Self-
reported functioning and well-being in patients with Parkinson’s
disease: Comparison of the Short-form Health Survey (SF-36) and the
Parkinson’s Disease Questionnaire (PDQ-39). Age Ageing 1995;24:
505–9.
[53] Fitzpatrick R, Peto V, Jenkinson C, Greenhall R, Hyman N. Health-
related quality of life in Parkinson’s disease: A study of outpatient
clinic attenders. Mov Disord 1997;12:916–22.
[54] Schrag A, Selai C, Jahanshahi M, Quinn NP. The EQ-5D—a generic
quality of life measure—is a useful instrument to measure quality of
life in patients with Parkinson’s disease. J Neurol Neurosurg
Psychiatry 2000;69:67–73.
[55] Badıa Llach X, Martınez Martın P. Quality of life measurement:
interest and applications. In: Martınez-Martın P, Koller WC, editors.
Quality of life in Parkinson’s disease. Barcelona: Masson, SA; 1999.
p. 17–36.
[56] Schrag A, Jahanshahi M, Quinn N. How does Parkinson’s disease
affect quality of life? A comparison with quality of life in the general
population Mov Disord 2000;15:1112–8.
[57] Pittock SJ, Mayr WT, McClelland RL, Jorgensen NW, Weigand SD,
Noseworthy JH, Rodriguez M. Quality of life is favourable for most
patients with multiple sclerosis. Arch Neurol 2004;61:679–86.
[58] Guarnaccia PJ. Anthropological perspectives: the importance of
culture in the assessment of quality of life. In: Spilker B, editor.
Quality of life and pharmacoeconomics in clinical trials. 2nd ed.
Philadelphia: Lippincot-Raven Publishers; 1996. p. 523–8.
[59] Ebersbach G, Sojer M, Muller J, Heijmenberg M, Poewe W.
Sociocultural differences in gait. Mov Disord 2000;15:1145–7.