Parkinson’s Disease Questionnaire-39

1. Abstract

The Parkinson’s Disease Questionnaire-39 (PDQ-39) is the international gold standard, disease-specific patient-reported outcome measure (PROM) developed to quantify subjective health status and health-related quality of life (HRQoL) among individuals diagnosed with Parkinson’s disease (PD). Conceptualized and validated by Viv Peto, Crispin Jenkinson, and Ray Fitzpatrick at the University of Oxford in 1995, the instrument addresses the multidimensional physical, psychological, cognitive, and social sequelae of idiopathic parkinsonism. Comprising 39 self-administered items, the questionnaire evaluates functional impairment and subjective distress across eight discrete domains: Mobility (10 items), Activities of Daily Living (6 items), Emotional Well-being (6 items), Stigma (4 items), Social Support (3 items), Cognitions (4 items), Communication (3 items), and Bodily Discomfort (3 items). Responses are captured using an ordinal five-point Likert scale ranging from 0 (Never) to 4 (Always / cannot do at all), with a standardized 4-week retrospective recall window.

Raw item responses within each subscale are transformed linearly into standardized domain scores ranging from 0 (indicating perfect health, with no self-reported difficulties) to 100 (reflecting maximum impairment). Furthermore, an aggregated metric known as the Parkinson’s Disease Questionnaire Summary Index (PDQ-39 SI) can be calculated by averaging the eight dimension scores to provide a global marker of disease burden. Extensive psychometric evaluations demonstrate high internal consistency (Cronbach’s alpha coefficients routinely exceeding 0.80 across functional domains), excellent test-retest reliability (intraclass correlation coefficients typically ranging between 0.76 and 0.93), robust convergent validity with clinician-rated measures such as the Movement Disorder Society-Unified Parkinson’s Disease Rating Scale (MDS-UPDRS) and the Hoehn and Yahr staging scale, and substantial responsiveness to pharmacological and neurosurgical interventions such as deep brain stimulation (DBS). Translated into more than 80 languages and validated across diverse sociocultural cohorts, the PDQ-39 remains the definitive benchmark instrument in both observational neurology research and randomized clinical trials.

2. Keywords

Parkinson’s Disease Questionnaire-39, PDQ-39, health-related quality of life, patient-reported outcome measure, neurodegenerative disorders, psychometrics, Parkinson’s disease, functional mobility, emotional well-being, stigma, movement disorders, clinimetrics

3. Authors

The development of the PDQ-39 was spearheaded by a multidisciplinary research team at the Health Services Research Unit, Department of Public Health and Primary Care, University of Oxford, United Kingdom:

• Viv Peto, MSc: Research Fellow in Health Services Research, Department of Public Health, University of Oxford. Primary researcher responsible for patient interview design, item pool generation, and initial clinimetric testing.
• Crispin Jenkinson, DPhil, MSc: Professor of Health Services Research and Director of the Health Services Research Unit, Nuffield Department of Population Health, University of Oxford. Leading authority on psychometric assessment, health status measurement, and questionnaire development methodologies.
• Ray Fitzpatrick, PhD: Professor of Public Health and Primary Care, Nuffield Department of Population Health, University of Oxford; Emeritus Fellow of Nuffield College. Renowned expert in medical sociology, patient-reported outcomes, and chronic disease evaluation.
• Ray C. Greenhall, DM, FRCP: Consultant Neurologist, Department of Neurology, Radcliffe Infirmary, Oxford. Clinical lead responsible for patient recruitment, clinical staging, and diagnostic verification during initial validation studies.
• Dutch Adaptation Lead: Astrid Verjee-Lorenz (1999), in collaboration with neurorehabilitation specialists in the Netherlands, conducted the translation, cross-cultural adaptation, and psychometric validation of the Dutch version.

Licensing and Distribution Inquiries: Oxford University Innovation Ltd., Clinical Outcomes, Buxton Court, 3 West Way, Oxford OX2 0JB, United Kingdom. Website: Oxford University Innovation Clinical Outcomes.

4. Purpose

Parkinson’s disease is a progressive, neurodegenerative movement disorder defined pathophysiologically by the loss of dopaminergic neurons in the substantia nigra pars compacta and the intraneuronal accumulation of misfolded alpha-synuclein. While traditional clinical paradigms have prioritized objective motor manifestations—specifically resting tremor, rigidity, bradykinesia, and postural instability—clinicians historically overlooked the profound burden imposed by non-motor features such as autonomic dysfunction, sleep disturbances, cognitive decline, mood disorders, and psychosocial alienation. The Parkinson’s Disease Questionnaire-39 was specifically formulated to redress this clinimetric gap by furnishing a rigorous, standardized, patient-derived instrument that captures the full spectrum of subjective illness experiences.

Clinical Applications

In routine neurological practice, the PDQ-39 functions as an essential diagnostic adjunct that bridges the perceptual divide between clinician assessments and patient lived experiences. While standard clinical staging systems—such as the Hoehn and Yahr scale or motor examinations via the MDS-UPDRS Part III—record physical signs during clinic visits, they frequently fail to detect transient motor fluctuations (“off” periods), dyskinesia-related exhaustion, concealed depressive states, or devastating social embarrassment. Regular clinical administration of the PDQ-39 enables:

• Systematic tracking of disease trajectory and functional deterioration over longitudinal follow-up intervals.
• Detection of concealed or stigmatizing non-motor complaints that patients may feel reluctant to raise during standard consultations, such as urinary embarrassment, conversational withdrawal, or panic regarding falls in public.
• Evaluation of subjective clinical benefit following therapeutic adjustments, including levodopa dose fractioning, catechol-O-methyltransferase (COMT) inhibitor initiation, continuous subcutaneous apomorphine infusion, or levodopa-carbidopa intestinal gel delivery.
• Pre- and post-operative assessment for advanced neurosurgical therapies, including bilateral subthalamic nucleus (STN) or globus pallidus internus (GPi) deep brain stimulation, where functional and psychological gains may diverge from purely motor outcomes.

Research Applications

Within clinical trial design, regulatory bodies—including the United States Food and Drug Administration (FDA) and the European Medicines Agency (EMA)—mandate the inclusion of validated patient-reported endpoints to substantiate clinical meaningfulness. The PDQ-39 serves as the premier primary or key secondary outcome measure in neuroprotective trials, symptomatic drug evaluations, physical therapy regimens, occupational interventions, and speech-language rehabilitation programs. By providing granular data across eight domains alongside an overarching Summary Index, the instrument enables researchers to determine whether an intervention yields targeted domain-specific gains (such as improved communication following voice treatment) or broad-spectrum quality of life improvements.

5. Psychological Construct

The central construct measured by the PDQ-39 is Health-Related Quality of Life (HRQoL) specific to Parkinson’s disease. HRQoL represents a complex, multi-tiered latent construct reflecting an individual’s subjective appraisal of their functional capacity, somatic comfort, emotional state, and interpersonal integration in the presence of a chronic, incurable disease process. The PDQ-39 decomposes this overarching construct into eight distinct yet intercorrelated latent dimensions:

1. Mobility (10 Items)

The Mobility domain assesses physical locomotion, spatial navigation, and gross motor autonomy. It addresses basic motor limitations (e.g., walking 100 yards or half a mile), home confinement, the psychological fear of public falling, and dependency on caregivers during transit. Deficits in this dimension reflect underlying axial rigidity, gait freezing, bradykinesia, and impaired postural reflexes, which directly restrict community ambulation and domestic engagement.

2. Activities of Daily Living (6 Items)

This subscale evaluates fine motor coordination, bilateral manual dexterity, and self-care efficacy. Items query difficulties encountered while bathing, dressing, manipulating buttons or shoe laces, writing legibly (capturing micrographic deterioration), cutting food, and drinking without spilling. This construct directly reflects the functional encroachment of action tremor, rigidity, and psychomotor slowness into fundamental self-maintenance routines, quantifying loss of personal autonomy.

3. Emotional Well-being (6 Items)

The Emotional Well-being dimension addresses the pervasive affective burden associated with neurodegenerative decline. Rather than capturing generic neuroses, it measures clinical dysphoria, existential distress, loneliness, tearfulness, anger, anxious hyperarousal, and anticipatory dread regarding progressive physical decrepitude. It reflects both the reactive psychological response to accumulating disability and the direct neurobiological sequelae of mesolimbic and noradrenergic pathway degeneration.

4. Stigma (4 Items)

The Stigma domain captures the perceived social discredit, self-directed shame, and public concealment behaviors resulting from noticeable physical symptoms. Items evaluate whether patients feel compelled to hide their diagnosis, avoid public eating or drinking, experience embarrassment due to their motor manifestations, or dread the gaze and reactions of strangers. This subscale models the social psychology of discrediting chronic conditions, tracing the trajectory from visible motor signs (such as tremor or dyskinesias) to secondary social withdrawal.

5. Social Support (3 Items)

This domain quantifies the patient’s subjective evaluation of closeness, emotional solidarity, and instrumental assistance received from immediate social networks. It specifically probes strain in primary intimate relationships and perceived lack of understanding or adequate support from spouses, life partners, and family members. It taps into relational strain, caregiver fatigue, and communicative alienation within the domestic unit.

6. Cognitions (4 Items)

The Cognitions subscale screens for daytime somnolence, working memory deficits, concentration difficulties, and neuropsychiatric phenomena. Specifically, it assesses unexpected daytime sleep episodes (sleep attacks), difficulties sustaining attention during intellectual tasks (such as reading or viewing television), memory lapses, and the occurrence of distressing hallucinations or vivid dreams. This subscale reflects frontostriatal network dysregulation, sleep architecture breakdown, and dopaminergic side effects.

7. Communication (3 Items)

This dimension operationalizes the social and linguistic friction caused by hypophonia, dysarthria, tachyphemia, and facial amimia (masked facies). Items query difficulty speaking clearly, feeling unable to express oneself effectively, and the distressing subjective perception of being ignored or overlooked by others in conversational environments. It measures the breakdown of communicative agency and expressive identity.

8. Bodily Discomfort (3 Items)

The Bodily Discomfort subscale isolates somatic distress stemming from musculoskeletal, dystonic, and autonomic disturbances. Items assess painful muscular cramps or spasms, generalized body and joint aches, and unpleasant thermoregulatory dysfunctions (such as excessive sweating or cold intolerance). This construct underscores the reality that Parkinson’s disease is fundamentally accompanied by severe sensory and autonomic discomforts that exacerbate functional disability.

6. Theoretical Framework

The conceptual architecture of the PDQ-39 is grounded in the intersection of Engel’s Biopsychosocial Model (1977) and the World Health Organization’s frameworks of functional status, initially formalized in the International Classification of Impairments, Disabilities, and Handicaps (ICIDH) and subsequently refined into the International Classification of Functioning, Disability and Health (ICF).

The Biopsychosocial Paradigm and WHO ICF Architecture

Traditional biomedical frameworks conceptualized Parkinson’s disease through a mechanistic lens: progressive dopaminergic nigrostriatal denervation produced pathognomonic physical impairments (rigidity, tremor, hypokinesia). The emergence of the biopsychosocial perspective established that disease manifestations cannot be evaluated in isolation from psychological coping mechanisms, interpersonal resources, and environmental constraints. In the ICF paradigm:

• Body Functions and Structures: The biological impairments of the basal ganglia produce sensory-motor deficits, captured partially by the Bodily Discomfort and Cognitions subscales.
• Activities: The immediate behavioral consequences of physical impairments, characterized by limitations in performing basic and complex actions, are represented by the Mobility and Activities of Daily Living subscales.
• Participation: The restrictions an individual encounters in societal and community contexts are captured by the Communication, Social Support, and Stigma subscales.

Sociological Theory of Chronic Illness and Stigmatization

The inclusion of the Stigma and Social Support domains is heavily influenced by Erving Goffman’s foundational sociological theory of social stigma (1963) and Michael Bury’s conceptualization of chronic illness as a “biographical disruption” (1982). Goffman posited that visible bodily anomalies or uncontrolled motor actions undermine an individual’s “social identity,” transforming them from a normal social actor to a discredited individual. In Parkinson’s disease, involuntary tremors, facial masking, drooling, or dyskinetic movements are frequently misconstrued by the general public as drunkenness, intellectual disability, or hostility.

Consequently, patients internalize this societal gaze, precipitating anticipated stigma, defensive concealment behaviors (e.g., hiding hands in pockets), and voluntary social isolation. Bury’s biographical disruption framework further illuminates how the gradual erosion of physical and communicative competencies ruptures expectations of self-sufficiency, strains marital bonds, and necessitates a continuous renegotiation of identity. The PDQ-39 operationalizes these complex sociological phenomena, ensuring that outcomes reflect personal and social identity alongside motor capability.

7. Validity

The psychometric validity of the PDQ-39 has been rigorously investigated across thousands of patients spanning early, intermediate, and advanced disease stages worldwide.

Content and Face Validity

Content validity was established through inductive item generation during initial instrument design. Peto, Jenkinson, and Fitzpatrick conducted exhaustive, in-depth, semi-structured qualitative interviews with individuals diagnosed with Parkinson’s disease across community and outpatient settings. Rather than relying on clinician-derived diagnostic criteria, the initial pool of candidate items was extracted directly from verbatim patient descriptions of their daily struggles. Subsequent draft iterations were evaluated by cognitive debriefing panels and neurology clinical panels to ensure comprehensive coverage of the disease spectrum.

Construct and Convergent Validity

Construct validity has been corroborated through extensive correlational testing against established clinician-rated scales and generic health status measures:

• Correlation with Clinical Motor Indices: The Mobility and Activities of Daily Living subscales exhibit strong, statistically significant correlations with the MDS-UPDRS Part II (Motor Experiences of Daily Living; r = 0.65 to 0.78) and Part III (Motor Examination; r = 0.52 to 0.68), as well as significant linear progressions across Hoehn and Yahr functional stages (Stages I through V; p < 0.001).
• Convergence with Generic Quality of Life Measures: When mapped against the Medical Outcomes Study 36-Item Short Form Survey (SF-36), the PDQ-39 Mobility subscale correlates strongly with SF-36 Physical Functioning (r = -0.70 to -0.82), while the PDQ-39 Emotional Well-being subscale demonstrates robust concordance with the SF-36 Mental Health domain (r = -0.68 to -0.76).
• Convergence with Affective Measures: The Emotional Well-being dimension demonstrates high convergent validity with formal psychiatric instruments, correlating strongly with the Beck Depression Inventory (BDI) (r = 0.68 to 0.74) and the Hospital Anxiety and Depression Scale (HADS) (r = 0.62 to 0.71).

Discriminant and Known-Groups Validity

The PDQ-39 demonstrates outstanding known-groups validity, successfully differentiating cohorts classified by motor complication phenotypes (e.g., patients experiencing motor fluctuations and unpredictable “off” periods versus stable responders, p < 0.001), fall histories (recurrent fallers versus non-fallers, p < 0.001), and disease duration strata. Furthermore, divergent validity is demonstrated by weak correlations between non-corresponding constructs; for instance, the Bodily Discomfort and Social Support subscales correlate poorly with objective cognitive tests such as the Mini-Mental State Examination (MMSE) (r < 0.20), confirming that distinct subscales measure divergent phenomena.

8. Reliability

The PDQ-39 demonstrates high reliability across both classic test theory metrics and modern clinimetric evaluations.

Internal Consistency

Across numerous international validation studies (including the original UK cohorts, the Dutch validation by Verjee-Lorenz, and extensive North American, European, and Asian cohorts), the instrument consistently achieves acceptable to excellent internal consistency:

• Mobility: Cronbach’s alpha (α) ranges between 0.89 and 0.93.
• Activities of Daily Living: α ranges between 0.85 and 0.91.
• Emotional Well-being: α ranges between 0.80 and 0.88.
• Stigma: α ranges between 0.78 and 0.85.
• Communication: α ranges between 0.72 and 0.82.
• Cognitions: α ranges between 0.65 and 0.76. (Slightly lower values are attributed to the multidimensional nature of items combining sleepiness, memory, and psychotic symptoms).
• Bodily Discomfort: α ranges between 0.71 and 0.78.
• Social Support: α ranges between 0.68 and 0.78. (Lower values in this three-item subscale are common, as partner and family support may dissociate).
• Summary Index (PDQ-39 SI): Global composite reliability consistently yields α values between 0.90 and 0.95.

Test-Retest Reliability and Measurement Error

When evaluated over stable retest intervals (spanning 7 to 28 days among clinically stable patients maintained on unchanged pharmacological regimens), test-retest reliability evaluated via Intraclass Correlation Coefficients (ICC) regularly exceeds 0.80 for the majority of domains. Typical reported ICC values include: Mobility (ICC = 0.88 – 0.94), ADL (ICC = 0.86 – 0.92), Emotional Well-being (ICC = 0.78 – 0.86), and the Summary Index (ICC = 0.89 – 0.94). The Standard Error of Measurement (SEM) and Minimal Detectable Change at the 95% confidence level (MDC₉₅) have been quantified as approximately 4.5 to 5.5 points for the PDQ-39 Summary Index, establishing that a change exceeding 5 points on the 100-point metric represents true clinical change beyond measurement noise.

9. Factor Analysis

The structural dimensionality of the PDQ-39 has been evaluated via exploratory factor analysis (EFA), confirmatory factor analysis (CFA), and Item Response Theory (IRT) / Rasch measurement models.

Exploratory Factor Analysis (EFA)

During original instrument validation, principal components analysis with varimax rotation of the 39 items consistently isolated eight factors with eigenvalues exceeding 1.0, accounting for more than 60% of the total variance. Items demonstrated high primary factor loadings (> 0.50) onto their designated subscales with modest secondary cross-loadings:

• Factor 1 (Mobility): Items 1 through 10 load strongly (loadings 0.58–0.84) on a primary physical ambulation component.
• Factor 2 (ADL): Items 11 through 16 demonstrate high loadings (0.62–0.81) onto fine motor self-care.
• Factor 3 (Emotional Well-being): Items 17 through 22 display prominent loadings (0.55–0.79) onto affective and anxious distress.
• Factor 4 (Stigma): Items 23 through 26 group tightly (loadings 0.68–0.85) around public embarrassment and concealment.
• Factors 5 through 8: Clearly isolate Social Support (items 27–29), Cognitions (items 30–33), Communication (items 34–36), and Bodily Discomfort (items 37–39).

Confirmatory Factor Analysis (CFA)

Subsequent international CFA investigations have tested both the orthogonal eight-factor model and a hierarchical second-order model (wherein the eight first-order factors load onto a general HRQoL latent dimension, justifying the calculation of the PDQ-39 Summary Index). Goodness-of-fit indices for the correlated eight-factor structure across large patient samples have established acceptable to good fit:

• Comparative Fit Index (CFI): Values generally range from 0.91 to 0.95.
• Tucker-Lewis Index (TLI): Typically ranges from 0.90 to 0.94.
• Root Mean Square Error of Approximation (RMSEA): Estimates range from 0.048 to 0.062 (90% CI [0.044, 0.066]), indicating satisfactory parameter fit.
• Standardized Root Mean Square Residual (SRMR): Routinely remains below 0.06.

Rasch and Modern Psychometric Critiques

Application of the Rasch measurement model has revealed some nuances in scale functioning. While the majority of subscales fulfill requirements of invariant item ordering and unidimensionality, specific items—such as Item 30 (“fallen asleep during the day”) and Item 33 (“distressing dreams or hallucinations”) within the Cognitions dimension—occasionally exhibit moderate item misfit (outfit/infit mean squares > 1.3), indicating that non-motor cognitive and psychiatric symptoms form a heterogeneous clinical cluster.

10. Instrument / Measurement Tool

• Formal Instrument Name: Parkinson’s Disease Questionnaire-39 (PDQ-39).
• Instrument Type: Disease-specific, self-administered patient-reported outcome measure (PROM) / health-related quality of life questionnaire. (Proxy/carer administration versions are available when severe cognitive impairment precludes self-completion).
• Target Population: Adults and older adults diagnosed with Parkinson’s disease across all stages of progression.
• Total Number of Items: 39 items.
• Recall Period: Past 4 weeks (one month).
• Subscale Structure (8 Dimensions):
  • Mobility: Items 1–10 (10 items).
  • Activities of Daily Living (ADL): Items 11–16 (6 items).
  • Emotional Well-being: Items 17–22 (6 items).
  • Stigma: Items 23–26 (4 items).
  • Social Support: Items 27–29 (3 items).
  • Cognitions: Items 30–33 (4 items).
  • Communication: Items 34–36 (3 items).
  • Bodily Discomfort: Items 37–39 (3 items).
• Response Format: Standard 5-point Likert scale:
  • 0 = Never
  • 1 = Occasionally
  • 2 = Sometimes
  • 3 = Often
  • 4 = Always (or cannot do at all)
• Scoring and Transformation Algorithm:
  • Raw Subscale Score: Sum the item responses within a designated subscale.
  • Transformed Subscale Score (0–100 Metric):
    
    Subscale Score = (Sum of actual item scores / Maximum possible score for that subscale) × 100
    
    Where maximum possible scores are calculated as the number of items multiplied by 4 (e.g., Mobility max = 10 × 4 = 40; ADL max = 6 × 4 = 24; Social Support max = 3 × 4 = 12).
  • Parkinson’s Disease Questionnaire Summary Index (PDQ-39 SI):
    
    PDQ-39 SI = (Sum of all 8 transformed dimension scores) / 8
  • Directionality: Higher scores denote greater impairment, elevated symptom burden, and poorer health-related quality of life. A score of 0 denotes optimal health status (no reported difficulty), while a score of 100 denotes total subjective impairment across assessed areas.
• Completion Duration: Approximately 8 to 15 minutes.

11. Permissions & Fee and Test Year

Publication Year: The English validation was published in 1995 (Peto, Jenkinson, Fitzpatrick, & Greenhall). The Dutch validation was published in 1999 (Verjee-Lorenz).

Copyright and Proprietary Rights: The copyright of the PDQ-39 is owned by Oxford University Innovation Ltd., the technology transfer organization of the University of Oxford. All rights are strictly reserved.

Licensing and Fee Structure: The PDQ-39 is not public domain. Access to the instrument, certified translations, and scoring algorithms requires execution of a formal licensing agreement through Oxford University Innovation’s Clinical Outcomes division:

• Academic Research & Clinical Practice: Royalty-free or nominal administrative licenses are routinely granted to publicly funded academic researchers, graduate student research projects, and non-commercial routine clinical care settings upon formal registration.
• Commercial & Industry Trials: Pharmaceutical firms, contract research organizations (CROs), and commercial device manufacturers must pay commercial licensing fees, user fees, and per-patient administration royalties.

Official Contact for Permissions: Clinical Outcomes Team, Oxford University Innovation Ltd., Buxton Court, 3 West Way, Oxford OX2 0JB, United Kingdom. E-mail: [email protected]; Web: https://innovation.ox.ac.uk/clinical-outcomes/.

12. References

• Bury, M. (1982). Chronic illness as a biographical disruption. Sociology of Health & Illness, 4(2), 167–182. https://doi.org/10.1111/1467-9566.ep11339939
• Engel, G. L. (1977). The need for a new medical model: A challenge for biomedicine. Science, 196(4286), 129–136. https://doi.org/10.1126/science.847460
• Fitzpatrick, R., Peto, V., Jenkinson, C., Greenhall, R., & Hyman, N. (1997). Health-related quality of life in Parkinson’s disease: A study of questionnaire components. Quality of Life Research, 6(6), 555–560. https://doi.org/10.1023/A:1018440013978
• Goffman, E. (1963). Stigma: Notes on the management of spoiled identity. Prentice-Hall.
• Jenkinson, C., Fitzpatrick, R., Norquist, J., Findley, L., & Hughes, A. (2003). Cross-cultural evaluation of the Parkinson’s Disease Questionnaire: Tests of data quality, score reliability, response standard errors, and scaling assumptions in the United States, Canada, and the United Kingdom. Journal of Clinical Epidemiology, 56(9), 843–847. https://doi.org/10.1016/S0895-4356(03)00155-2
• Jenkinson, C., Peto, V., Fitzpatrick, R., Greenhall, R., & Hyman, N. (1995). 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 and Ageing, 24(6), 505–509. https://doi.org/10.1093/ageing/24.6.505
• Martinez-Martin, P., Jeukens-Visser, M., Lyons, K. E., Rodriguez-Blazquez, C., Selai, C., Siderowf, A., Welsh, M., Poewe, W., Rascol, O., Sampaio, C., Stebbins, G. T., & Goetz, C. G. (2011). Health-related quality-of-life scales in Parkinson’s disease: Critique and recommendations. Movement Disorders, 26(13), 2371–2380. https://doi.org/10.1002/mds.23834
• Peto, V., Jenkinson, C., Fitzpatrick, R., & Greenhall, R. (1995). The development and validation of a short measure of functioning and well being for individuals with Parkinson’s disease. Quality of Life Research, 4(3), 241–248. https://doi.org/10.1007/BF02260863
• Peto, V., Jenkinson, C., & Fitzpatrick, R. (1998). PDQ-39: A review of the development, validation and application of a Parkinson’s disease quality of life questionnaire and its associated short form. Current Medical Research and Opinion, 14(4), 231–241. https://doi.org/10.1185/03007999809114251
• Verjee-Lorenz, A. (1999). Validatie van de Nederlandse versie van de Parkinson’s Disease Questionnaire (PDQ-39). Department of Neurology, Academic Medical Center, Amsterdam.

13. Items of the Scale