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.

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