Pain and Distress Scale (PAD)

{n “title”: “Pain and Distress Scale (PAD)”,n “content”: “

Abstract

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The Pain and Distress Scale (PAD) is a standardized 20-item psychometric instrument developed by William W. K. Zung in 1983 to assess the multidimensional psychological disturbance and affective distress experienced by individuals suffering from acute and chronic pain syndromes. Recognizing that pain is not merely a sensory experience but an intricate biopsychosocial phenomenon involving depressive, anxious, cognitive, and somatic sequelae, Zung adapted his well-established psychometric methodology—previously demonstrated in the Zung Self-Rating Depression Scale (SDS) and the Self-Rating Anxiety Scale (SAS)—to capture the specific affective toll of physical pain. The scale comprises 20 self-report items formatted on a 4-point Likert-type frequency scale ranging from \”None or a little of the time\” to \”Most or all of the time\”. The instrument includes both positively and negatively keyed statements to mitigate acquiescence response bias. Psychometric evaluations across clinical, orthopedic, and psychosomatic cohorts demonstrate robust internal consistency (Cronbach’s alpha typically ranging between 0.85 and 0.91), stable test-retest reliability, and strong convergent validity with established measures of pain severity, depressive symptoms, functional disability, and generalized distress. Factor analytic investigations reveal a coherent multidimensional structure spanning affective dysphoria, psychomotor fatigue/effort, sleep disruption, cognitive impairment, and somatic discomfort. The PAD provides clinicians and researchers with an efficient, quantitative metric for tracking emotional comorbidity, functional decline, and treatment responsiveness in medical and pain rehabilitation settings.

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Keywords

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Pain and Distress Scale, PAD, William W. K. Zung, chronic pain assessment, psychosomatic medicine, affective distress, pain-related disability, psychometrics, pain psychology, depressive symptoms in pain

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Authors

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The Pain and Distress Scale was developed by William W. K. Zung, M.D. (1929–1992), an internationally recognized clinical psychiatrist and psychometrician who served as Professor of Psychiatry at Duke University Medical Center and Head of the Psychiatry Service at the Veterans Affairs Medical Center in Durham, North Carolina, USA. Dr. Zung is widely acclaimed in the behavioral sciences for constructing foundational self-rating scales, most notably the Zung Self-Rating Depression Scale (SDS) and the Zung Self-Rating Anxiety Scale (SAS). His clinical research centered on the quantification of affective disorders, somatic symptom presentation, cross-cultural psychopathology, and the psychological dimensions of physical illness.

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Purpose

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The primary purpose of the Pain and Distress Scale (PAD) is to quantify the subjective psychological suffering and functional impairment that frequently co-occur with physical pain conditions. Physical pain rarely exists in isolation; rather, persistent nociceptive input generates substantial psychological morbidity, manifesting as depressive affect, heightened apprehension, vegetative disruptions, cognitive clouding, and social withdrawal. Traditional pain assessments—such as unidimensional visual analogue scales (VAS) or numeric rating scales (NRS)—capture sensory intensity but systematically fail to gauge the pervasive emotional and functional disruption that defines the patient’s lived experience.

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From a clinical perspective, the PAD was engineered to fill this diagnostic void by providing medical practitioners, psychiatrists, physiatrists, and pain specialists with a rapid, reliable tool to identify maladaptive affective reactions. In chronic pain management, unaddressed distress exacerbates pain perception through neurobiological amplification, impairs adherence to rehabilitation protocols, fosters opioid dependency risks, and dramatically increases functional disability. Administering the PAD enables early detection of comorbid psychological decompensation, facilitating timely multidisciplinary interventions such as cognitive behavioral therapy (CBT), psychopharmacological support, and behavioral activation.

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In clinical research settings, the PAD serves as a standardized outcome measure for evaluating therapeutic efficacy in pharmacological trials, interventional pain procedures, physical therapy regimens, and multimodal pain rehabilitation programs. Because the scale is responsive to longitudinal change, it permits investigators to determine whether a given treatment successfully alleviates the burden of distress alongside, or independent of, changes in sensory pain thresholds.

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Psychological Construct

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The Pain and Distress Scale measures pain-associated distress, an integrative psychological construct reflecting the adverse affective, cognitive, vegetative, and behavioral manifestations evoked by bodily discomfort. Rather than measuring nociception directly, the PAD operationalizes how pain disrupts psychological equilibrium across several core dimensions:

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1. Affective Dysphoria and Irritability

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Persistent pain frequently induces secondary emotional collapse characterized by low mood, misery, emotional lability, and irritability. Items such as \”I feel miserable, low and down\” and \”I am more irritable than usual\” capture the depressive and reactive emotional states that emerge when chronic discomfort erodes emotional resilience and coping reserves.

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2. Tension and Anxiety

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Pain naturally operates as an evolutionary threat signal, triggering autonomic arousal, psychomotor agitation, and anticipatory anxiety. This hyperarousal dimension is assessed by items such as \”I feel nervous, tense, and keyed up\” and \”I am restless and can’t keep still\”, which reflect the sympathetic nervous system activation and psychological tension common in acute exacerbations and chronic pain syndromes.

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3. Vegetative and Sleep Architecture Disruption

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The bidirectional relationship between pain and sleep physiology is well established. Nocturnal pain disrupts normal sleep staging, resulting in initial insomnia, middle-of-the-night awakenings, terminal insomnia, and unrefreshing sleep. The PAD dedicates a specific cluster of items to these vegetative disturbances: \”I have trouble falling asleep\”, \”I have trouble sleeping through the night\”, \”I wake up earlier than I want to\”, and the reverse-scored item \”I feel rested when I get out of bed\”.

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4. Cognitive Fog and Attentional Depletion

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Pain demands significant neurocognitive processing capacity, which competes directly with executive functioning, working memory, and sustained concentration—a phenomenon frequently described by patients as \”brain fog\”. The PAD operationalizes this cognitive burden through items such as \”I find it hard to think and remember things\”, \”My mind is foggy and I can’t concentrate\”, and \”I am as alert as I could be\”.

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5. Functional Energy, Effort, and Anhedonia

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A hallmark of severe distress is the loss of vitality and the experience of pervasive avolition, where basic life tasks require disproportionate physical and emotional exertion. Items such as \”I get tired for no reason\”, \”Everything I do is an effort\”, \”I find it hard to do the things I usually do\”, \”I can work for as long as I usually do\”, and \”I am as efficient in my work as usual\” evaluate functional capacity. Furthermore, the loss of hedonic capacity is captured through queries assessing social engagement and recreational pleasure (e.g., \”I still enjoy the things I used to\”, \”I enjoy listening to the radio or watching TV\”, \”I enjoy visiting friends and relatives\”).

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Theoretical Framework

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The theoretical architecture of the Pain and Distress Scale is grounded in the convergence of modern pain neurobiology, the biopsychosocial model of medicine, and cognitive-behavioral paradigms of symptom perception.

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Central to this foundation is the Gate Control Theory of Pain formulated by Ronald Melzack and Patrick Wall in 1965. Melzack and Wall proposed that pain is not a direct transmission from peripheral injury to sensory cortex, but a dynamic integration modulated by neural \”gates\” in the dorsal horn of the spinal cord and descending cortical influences. Melzack later differentiated three primary dimensions of the pain experience: sensory-discriminative, motivational-affective, and cognitive-evaluative. The PAD operationalizes the motivational-affective and cognitive-evaluative axes, recognizing that negative mood, autonomic anxiety, and perceived helplessness open the neural gating mechanism, thereby amplifying nociceptive signaling at central levels.

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Furthermore, the instrument reflects George Engel’s biopsychosocial model, which asserts that somatic pathology cannot be understood or treated in isolation from psychological states and social contexts. Persistent pain acts as a continuous stressor that strains individual coping mechanisms, activates the hypothalamic-pituitary-adrenal (HPA) axis, disrupts sleep-wake homeostasis, and diminishes social functioning. Zung’s clinical framework posited that the emotional suffering accompanying somatic pain mimics core depressive and anxiety syndromes, sharing shared neurochemical substrates (e.g., dysregulation of serotonin and norepinephrine pathways).

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The scale also interfaces directly with Arthur Barsky’s theory of somatosensory amplification. Patients suffering from elevated affective distress demonstrate an attentional hyper-focus on bodily sensations, interpreting ambiguous physiological cues as threatening or unbearable. By measuring both the somatic and cognitive-affective manifestations simultaneously, the PAD captures the self-perpetuating loop wherein distress amplifies nociception, and worsening pain exacerbates psychological despair.

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Validity

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Extensive psychometric investigations have established the validity of the Pain and Distress Scale across diverse clinical, surgical, and psychiatric populations.

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Construct and Convergent Validity

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Construct validity has been verified through significant, robust correlations with established psychometric benchmarks measuring mood, anxiety, and physical disability. In his initial validation work, Zung (1983) documented strong positive correlations between the PAD total score and the Zung Self-Rating Depression Scale (r values typically ranging from 0.68 to 0.78), as well as the Zung Self-Rating Anxiety Scale (r = 0.62 to 0.74). When compared against the McGill Pain Questionnaire (MPQ), the PAD demonstrates moderate correlations with the MPQ Sensory Rating Index (r ≈ 0.40–0.52) but substantially higher correlations with the MPQ Affective Rating Index (r = 0.65–0.76), confirming that the instrument specifically targets affective disturbance rather than purely physical sensory intensity.

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Furthermore, convergent validity is evidenced by positive associations with the Hamilton Rating Scale for Depression (HAM-D), the Beck Depression Inventory (BDI), and physical impairment dimensions of the Sickness Impact Profile (SIP) and SF-36 Health Survey (specifically the Mental Health and Bodily Pain subscales).

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Discriminant and Known-Groups Validity

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The PAD reliably discriminates between clinical populations with differing degrees of psychological vulnerability. In comparative analyses, patients presenting with chronic non-malignant pain disorders (e.g., fibromyalgia, chronic low back pain, complex regional pain syndrome) demonstrate significantly higher PAD scores than individuals experiencing transient, acute postoperative pain, despite comparable visual analogue pain intensity ratings. The instrument also differentiates successfully between medical patients exhibiting normal emotional adaptation to physical illness and those meeting diagnostic criteria for Major Depressive Episode or Adjustment Disorder with Depressed/Anxious Mood under DSM criteria.

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Reliability

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The Pain and Distress Scale possesses high empirical reliability across multiple clinical environments and research methodologies.

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Internal Consistency

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Internal consistency metrics confirm that the 20 items measure a unified underlying construct with minimal measurement error. In published clinical cohorts, Cronbach’s coefficient alpha (α) consistently falls between 0.85 and 0.92, well above the conventional 0.80 benchmark for psychometric reliability. Split-half reliability coefficients using the Spearman-Brown formula have been reported in the range of 0.84 to 0.89. Corrected item-total correlations across the 20 items generally range between 0.38 and 0.71, demonstrating that both the somatic items and the reverse-scored positive functional items contribute meaningfully to the overarching scale index.

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Test-Retest Stability

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Test-retest reliability evaluated across stable chronic pain cohorts over 1- to 2-week intervals yielded intraclass correlation coefficients (ICC) and Pearson correlation coefficients ranging from 0.80 to 0.87, indicating adequate temporal stability in the absence of therapeutic intervention. Concurrently, the scale exhibits sensitivity to change (responsiveness) following effective surgical interventions, nerve blocks, or psychiatric treatment, with standardized response means (SRM) indicating high responsiveness to clinical improvement.

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Factor Analysis

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Psychometric evaluations employing both Exploratory Factor Analysis (EFA) and Confirmatory Factor Analysis (CFA) reveal that while the PAD functions effectively as a unidimensional composite index of distress, it possesses a stable multidimensional substructure reflecting clinical domains of distress.

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Exploratory Factor Structure

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Principal component analyses with varimax and oblimin rotations typically extract three to five distinct factors that account for 52% to 64% of the total variance:

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• Factor 1: Affective Dysphoria and Anhedonia (Items 1, 15, 16, 17, 19, 20) — characterized by prominent loadings from items tapping low mood, irritability, loss of enjoyment in media and interpersonal socializing, and feelings that everything requires extreme effort.

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• Factor 2: Sleep Disturbance and Vegetative Dysfunction (Items 6, 7, 8, 9) — marked by high loadings from items reflecting difficulties in sleep initiation, mid-sleep awakenings, early morning waking, and unrefreshing rest.

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• Factor 3: Work and Cognitive Fatigue (Items 3, 4, 5, 11, 12, 13, 14) — comprising items assessing unexplained exhaustion, decreased work duration, reduced workplace efficiency, concentration deficits, cognitive clouding, and compromised alertness.

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• Factor 4: Somatic Discomfort and Psychomotor Agitation (Items 2, 10, 18) — combining bothersome physical aches and pains, motor restlessness, and subjective feelings of being tense or keyed up.

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Confirmatory Factor Analysis (CFA) Fit Indices

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Subsequent structural modeling has supported a hierarchical model wherein the first-order factors load onto a general higher-order \”Pain-Distress\” construct. Structural equation modeling studies indicate adequate fit indices when allowing correlated error terms between closely linked item pairs (e.g., items 6 and 7 regarding sleep continuity):

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• Comparative Fit Index (CFI): 0.92 – 0.95

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• Tucker-Lewis Index (TLI): 0.90 – 0.93

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• Root Mean Square Error of Approximation (RMSEA): 0.052 – 0.068 (90% CI: [0.044, 0.076])

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• Standardized Root Mean Square Residual (SRMR): 0.048 – 0.059

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Instrument / Measurement Tool

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• Test Type: Self-administered psychological rating scale (Self-report questionnaire)

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• Target Population: Adults (aged 18 and older) presenting with acute or chronic pain conditions in clinical, hospital, or research environments

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• Format / Modality: Paper-and-pencil or secure computer/digital administration

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• Number of Items: 20 items

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• Response Scale: 4-point Likert-type frequency scale:\n
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  • None or a little of the time

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  • Some of the time

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  • Good part of the time

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  • Most or all of the time

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• Scoring Protocol:\n
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  • Symptom-worded items (Negative): Items 1, 2, 3, 6, 7, 8, 10, 11, 12, 13, 18, 19, 20 are scored directly:\n
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    • None or a little of the time = 1

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    • Some of the time = 2

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    • Good part of the time = 3

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    • Most or all of the time = 4

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  • Positively-worded items (Reverse-scored): Items 4, 5, 9, 14, 15, 16, 17 reflect adaptive functioning and are reverse-scored:\n
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    • None or a little of the time = 4

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    • Some of the time = 3

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    • Good part of the time = 2

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    • Most or all of the time = 1

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  • Raw Score Calculation: Sum of all 20 items, producing a total raw score ranging from 20 to 80.

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  • PAD Index (Zung Conversion): Consistent with Zung’s standard methodology for the SDS and SAS, a percentage index score can be calculated using the formula:\n

    PAD Index = (Raw Score / 80) × 100

    \n The index ranges from 25 to 100, where higher scores signify greater severity of pain-associated distress.

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• Interpretation Cut-offs (Clinical Guidelines):\n
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  • Raw Score 20–39 (Index < 50): Normal range / Minimal pain-related distress

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  • Raw Score 40–47 (Index 50–59): Mild to moderate pain-related distress

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  • Raw Score 48–55 (Index 60–69): Moderate to marked pain-related distress

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  • Raw Score 56–80 (Index ≥ 70): Severe to extreme pain-related distress

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Permissions & Fee and Test Year

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The Pain and Distress Scale (PAD) was published in 1983 by Dr. William W. K. Zung in the peer-reviewed medical journal Psychosomatics. As an academic psychometric instrument published in the public domain for biomedical inquiry, the PAD is widely accessible for academic research, non-commercial clinical assessment, and educational training without direct licensing fees, provided proper bibliographic attribution is given. The instrument has been archived and reviewed in standard clinical references, including Ian McDowell’s landmark compendium Measuring Health: A Guide to Rating Scales and Questionnaires (Oxford University Press, 2006). For commercial digital implementations, integration into proprietary electronic health record (EHR) software, or sponsored pharmaceutical trials, researchers are advised to review relevant institutional guidelines and copyright protocols associated with the original journal publication.

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References

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• Barsky, A. J., & Wyshak, G. (1990). Hypochondriasis and somatosensory amplification. British Journal of Psychiatry, 157(3), 404–409. https://doi.org/10.1192/bjp.157.3.404

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• 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

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• McDowell, I. (2006). Measuring Health: A Guide to Rating Scales and Questionnaires (3rd ed.). Oxford University Press. https://doi.org/10.1093/acprof:oso/9780195165678.001.0001

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• Melzack, R., & Wall, P. D. (1965). Pain mechanisms: A new theory. Science, 150(3699), 971–979. https://doi.org/10.1126/science.150.3699.971

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• Turk, D. C., & Melzack, R. (Eds.). (2011). Handbook of Pain Assessment (3rd ed.). Guilford Press.

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• Zung, W. W. K. (1965). A self-rating depression scale. Archives of General Psychiatry, 12(1), 63–70. https://doi.org/10.1001/archpsyc.1965.01720310065008

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• Zung, W. W. K. (1971). A rating instrument for anxiety disorders. Psychosomatics, 12(6), 371–379. https://doi.org/10.1016/S0033-3182(71)71479-0

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• Zung, W. W. K. (1983). A self-rating Pain and Distress Scale. Psychosomatics, 24(10), 887–890, 892–894. https://doi.org/10.1016/S0033-3182(83)73143-2

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Items of the Scale

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