Checklist Individual Strength

1. Abstract

The Checklist Individual Strength (CIS, often designated as the CIS-20) is a premier multidimensional self-report questionnaire designed to measure chronic fatigue and its behavioral and cognitive sequelae across diverse clinical and occupational populations. Developed in the Netherlands by Jan H. M. M. Vercoulen and colleagues (1994), the instrument addresses the conceptual limitation of treating fatigue as a monolithic, unidimensional phenomenon. The CIS comprises 20 items evaluated over a two-week recall period using an authentic 7-point Likert response scale ranging from 1 (“Yes, that is true”) to 7 (“No, that is not true”). Psychometric evaluations consistently confirm a four-factor orthogonal structure capturing: (a) Subjective Feeling of Fatigue (8 items), reflecting the core visceral and perceived experience of exhaustion; (b) Concentration (5 items), capturing subjective cognitive impairment and executive disruption; (c) Motivation (4 items), indexing abulia, drive, and initiation of pleasant activities; and (d) Physical Activity (3 items), measuring behavioral performance and daily physical output.

Extensive psychometric investigations among patients with chronic fatigue syndrome (CFS/ME), multiple sclerosis (MS), cancer-related fatigue, chronic obstructive pulmonary disease (COPD), rheumatoid arthritis, as well as non-clinical working adults, have demonstrated high internal consistency. Cronbach’s alpha coefficients routinely range from .83 to .93 for the total scale and subjective fatigue subscale, with subscale reliabilities generally exceeding .80. Test-retest reliability across clinical intervals demonstrates robust stability (intraclass correlation coefficients typically > .80 in stable cohorts). Construct, convergent, and discriminant validities are established via expected correlations with actigraphic motor output, biological markers of physical dysfunction, depression scales, and measures of functional impairment. A score of 35 or higher on the Subjective Fatigue subscale is widely utilized across empirical literature and clinical trials as a validated clinical cut-off indicating severe fatigue. The CIS remains one of the international gold standards for quantifying multidimensional fatigue outcomes in interventional research, behavioral medicine, and occupational health.

2. Keywords

Checklist Individual Strength, CIS-20, fatigue assessment, chronic fatigue syndrome, subjective fatigue, concentration impairment, motivation deficit, physical activity, psychometric validation, multidimensional fatigue scale

3. Authors

The Checklist Individual Strength was conceptualized, operationalized, and psychometrically validated by an interdisciplinary consortium of clinical researchers, immunologists, and medical psychologists at Radboud University Nijmegen Medical Centre (formerly University Hospital Nijmegen), Nijmegen, The Netherlands:

• Jan H. M. M. Vercoulen, Ph.D. — Department of Medical Psychology and Pulmonary Diseases, Radboud University Medical Centre, Nijmegen, The Netherlands. Specialized in behavioral medicine, chronic illness adaptation, and instrument development.
• C. M. A. Swanink, M.D., Ph.D. — Department of Medical Microbiology, Radboud University Medical Centre, Nijmegen, The Netherlands. Researched biological correlates, immunology, and infectious triggers of chronic fatiguing illnesses.
• J. F. M. Fennis, M.D., Ph.D. — Department of General Internal Medicine, Radboud University Medical Centre, Nijmegen, The Netherlands. Focus on somatic differential diagnostics in prolonged exhaustion.
• J. M. D. Galama, Ph.D. — Department of Medical Microbiology and Clinical Virology, Radboud University Medical Centre, Nijmegen, The Netherlands.
• Jos W. M. van der Meer, M.D., Ph.D., FRCP — Professor of Medicine, Department of General Internal Medicine, Radboud University Medical Centre, Nijmegen, The Netherlands. Leading international investigator in systemic inflammation, chronic infectious diseases, and post-viral fatigue syndromes.
• Gijs Bleijenberg, Ph.D. — Professor Emeritus of Medical Psychology, Expert Centre for Chronic Fatigue (ECCF), Radboud University Medical Centre, Nijmegen, The Netherlands. Renowned for pioneer cognitive-behavioral theoretical models and evidence-based interventions for persistent fatigue.

4. Purpose

The fundamental purpose of the Checklist Individual Strength is to provide an empirically grounded, nuanced, and psychometrically robust operationalization of fatigue that distinguishes phenomenological exhaustion from comorbid psychopathology such as major depressive disorder or general physical disability. Fatigue is among the most ubiquitous complaints encountered in primary care, rheumatology, oncology, neurology, and occupational medicine. However, historically, clinical inquiry suffered from instruments that collapsed mental fatigue, physical exhaustion, functional disability, and negative affect into undifferentiated composite metrics. The CIS was created specifically to resolve these epistemological and clinical ambiguities by isolating subjective weariness from its distinct functional manifestations.

The primary aims of the instrument encompass:

• Multidimensional Deconstruction: Disentangling the core subjective perception of lethargy from cognitive slowing, reduced volitional motivation, and objective reduction in behavioral activity. This differentiation is crucial because therapeutic interventions may improve subjective energy without immediately normalizing executive processing or physical mobility, and vice versa.
• Discriminative Diagnostics: Providing empirically verified cut-off scores that differentiate normal physiological tiredness resulting from acute exertion from disabling, pathological fatigue characteristic of conditions such as myalgic encephalomyelitis/chronic fatigue syndrome, post-cancer fatigue, and autoimmune diseases.
• Sensitivity to Longitudinal Change: Serving as a responsive primary or secondary endpoint in randomized controlled trials (RCTs) investigating cognitive behavioral therapy (CBT), graded exercise therapy (GET), pharmacological agents, and vocational rehabilitation protocols.
• Evaluation of Daily Occupational Impairment: Measuring the fatigue burden within industrial and occupational settings to detect impending workplace burnout, presenteeism, prolonged sick leave, and loss of work productivity.

The CIS incorporates a two-week recall window, making it less susceptible to ephemeral diurnal fluctuations than momentary ecological assessments while avoiding the retrospective recall biases inherent in instruments requiring six-month appraisals. The theoretical rationale emphasizes that chronic fatigue is perpetuated by a vicious cycle of cognitive attributions, altered somatic perception, and behavioral dysregulation, necessitating an instrument capable of measuring these distinct domains in parallel.

5. Psychological Construct

The Checklist Individual Strength conceptualizes fatigue not as an isolated somatic sensation, but as a complex multidimensional construct operating across affective, physiological, cognitive, and behavioral domains. The scale captures 20 items distributed across four robust latent dimensions:

1. Subjective Feeling of Fatigue (8 items)

This subscale assesses the respondent’s primary, direct sensory and phenomenological experience of tiredness, exhaustion, and physical depletion. It addresses both the ease with which fatigue is triggered and the inadequacy of physiological restoration (e.g., waking unrefreshed). Representing the core axis of chronic fatigue syndromes, this subscale quantifies how physically drained or vulnerable an individual perceives their body to be. Items measuring this construct include evaluations of feeling tired, physically exhausted, weak, rested, or in bad physical condition.

2. Concentration (5 items)

Cognitive fatigue is frequently experienced as distinct from somatic weakness, characterized by disruptions in executive functioning, sustained attention span, working memory allocation, and mental processing speed. The Concentration subscale captures the subjective cognitive friction or effort required to execute routine intellectual tasks. Patients experiencing high cognitive fatigue report that thinking requires excessive mental energy, that thoughts easily drift or scatter, and that sustained focus cannot be maintained during vocational or domestic tasks.

3. Motivation (4 items)

Motivation in the context of the CIS captures intrinsic volitional drive, hedonic engagement, and the spontaneous generation of goal-directed behavioral plans. In individuals suffering from persistent fatigue, severe abulia or apathy may emerge not necessarily from primary clinical depression, but from a persistent depletion of energetic resources. This subscale measures the desire to initiate novel activities, execute pleasant recreational tasks, and formulate daily plans, distinguishing between somatic inability and avolitional apathy.

4. Physical Activity (3 items)

The Physical Activity subscale evaluates behavioral manifestations and output over the preceding fortnight. It captures the subjective assessment of how much an individual accomplishes and whether their daily motor behavior is severely curtailed. Unlike objective actigraphy, which records raw kinetic acceleration, this dimension measures the self-perceived deficit in productivity and daily task execution, reflecting the behavioral accommodation to chronic energetic depletion.

6. Theoretical Framework

The theoretical architecture undergirding the CIS is anchored in the cognitive-behavioral perpetuation model of chronic fatigue developed by Bleijenberg, Vercoulen, and colleagues at the Nijmegen Expert Centre for Chronic Fatigue. This conceptual framework posits a clear distinction between the predisposing factors, precipitating triggers, and perpetuating mechanisms of persistent exhaustion.

While an acute viral infection, major life stressor, or somatic illness may initiate an episode of profound exhaustion (precipitating factors), the transition into chronic, recalcitrant fatigue is mediated by cognitive, affective, and behavioral loops:

• Cognitive Attributions and Somatosensory Amplification: Patients often adopt somatic attributions for all physiological sensations, interpreting normal post-exertional muscular sensations as catastrophic signs of underlying bodily damage or unhealed pathology. This somatosensory focus amplifies the perceived intensity of subjective fatigue.
• Behavioral Dysregulation: Individuals frequently oscillate between two dysfunctional behavioral patterns: severe passive avoidance (prolonged bed rest and immobility leading to secondary physical deconditioning) or an unstable “boom-and-bust” pattern (overexertion on lower-symptom days followed by dramatic crashes and prolonged bed rest).
• Loss of Perceived Control: As social, occupational, and recreational pursuits are progressively abandoned, individuals experience diminished self-efficacy and low motivation, which further deepens cognitive weariness and subjective exhaustion.

By measuring Subjective Fatigue, Concentration, Motivation, and Physical Activity independently, the CIS operationalizes this cognitive-behavioral model. It allows clinicians and investigators to determine whether a patient presents with primary behavioral deconditioning (low physical activity with high subjective fatigue) or cognitive-volitional burnout (severely impaired concentration and motivation with preserved baseline activity).

7. Validity

The psychometric validity of the Checklist Individual Strength has been scrutinized in dozens of international studies across varied populations:

• Construct and Factorial Validity: Exploratory and confirmatory factor analyses repeatedly validate the four-factor orthogonal model. Across distinct medical cohorts—such as post-cancer survivors, multiple sclerosis patients, and individuals with rheumatoid arthritis—the 20 items consistently demonstrate strong convergent loadings (> .50) onto their designated latent constructs without significant cross-loadings.
• Convergent Validity: The Subjective Fatigue subscale correlates robustly with other validated measures of physical and mental exhaustion, including the Chalder Fatigue Scale (r = .70 to .82), the Fatigue Severity Scale (FSS; r = .68 to .78), and the Vitality subscale of the SF-36 Health Survey (r = -.75 to -.84). Furthermore, the Concentration subscale exhibits moderate-to-strong correlations with neuropsychological self-report inventories and subjective cognitive failure metrics.
• Discriminant Validity: The CIS successfully differentiates fatigue from clinical depression and anxiety. Although fatigue and affective disorders share some phenotypic variance, empirical investigations utilizing the Beck Depression Inventory (BDI) and Hospital Anxiety and Depression Scale (HADS) show that the CIS subscales—particularly Physical Activity and Concentration—diverge meaningfully from affective dysphoria, confirming that fatigue is measured as an independent construct rather than a mere proxy for depressed mood.
• Predictive and Ecological Validity: The CIS Physical Activity subscale correlates moderately with objective accelerometer and actometer counts in chronic fatigue cohorts (r = .35 to .50), reflecting real-world behavioral output. Crucially, high baseline scores on the Subjective Fatigue and Concentration subscales prospectively predict prolonged work absenteeism, delayed return to work, and elevated utilization of healthcare services.
• Clinical Cut-Offs: A cutoff score of ≥ 35 on the Subjective Fatigue subscale represents severe fatigue. Scores between 27 and 34 indicate heightened fatigue, whereas scores ≤ 26 are typical for healthy, non-fatigued community samples.

8. Reliability

The reliability parameters of the CIS demonstrate high internal consistency and longitudinal measurement precision:

• Internal Consistency: In the seminal validation study by Vercoulen et al. (1994) comprising healthy controls and patients meeting international criteria for chronic fatigue syndrome, Cronbach’s alpha coefficients were established as follows:
  • Total CIS Scale: α = .90 – .93
  • Subjective Feeling of Fatigue (8 items): α = .88 – .93
  • Concentration (5 items): α = .82 – .88
  • Motivation (4 items): α = .73 – .83
  • Physical Activity (3 items): α = .78 – .87

  Subsequent studies involving cancer survivors (Servaes et al., 2002) and working populations (Bültmann et al., 2000) have reaffirmed alphas > .80 for all subscales.

• Test-Retest Stability: In untreated, clinically stable cohorts assessed across a two- to four-week interval, intraclass correlation coefficients (ICCs) and Pearson correlation coefficients consistently exceed r = .80, indicating that the instrument is not unduly perturbed by minor day-to-day fluctuations while reliably capturing persistent symptom burdens.
• Measurement Invariance: Cross-validation studies evaluating multi-group invariance have confirmed metric and scalar invariance across biological sex and various age brackets, verifying that observed score differences reflect true variations in latent fatigue dimensions rather than differential item functioning.

9. Factor Analysis

The structural integrity of the CIS was initially determined using principal component analysis (PCA) with varimax rotation by Vercoulen and colleagues (1994), which yielded four distinct eigenvalues greater than 1.0, accounting for approximately 64% to 68% of the total item variance.

Subsequent confirmatory factor analyses (CFA) have systematically evaluated alternative structural models:

• Unidimensional Model: A single global fatigue factor yields poor fit indices across all clinical cohorts (RMSEA > .12, CFI < .80), demonstrating that fatigue cannot be modeled accurately as a singular homogeneous continuum.
• Four-Factor Correlated Model: A four-factor structure (Subjective Fatigue, Concentration, Motivation, Physical Activity) achieves superior model fit across empirical studies:
  • Root Mean Square Error of Approximation (RMSEA): .048 – .062 (indicating close approximate model fit)
  • Comparative Fit Index (CFI): .94 – .97
  • Tucker-Lewis Index (TLI): .93 – .96
  • Standardized Root Mean Square Residual (SRMR): < .055

Standardized factor loadings across studies are robust. For the Subjective Fatigue factor, item loadings range from .62 to .86 (e.g., “I feel tired” and “Physically I feel exhausted” routinely exceed .80). For the Concentration dimension, factor loadings span from .65 to .84. The Motivation items load between .58 and .79, and the Physical Activity items demonstrate loadings between .64 and .88. Inter-factor correlations among the four latent constructs typically range between r = .30 and r = .65, affirming that while the subscales share common variance associated with the overarching syndrome of exhaustion, they preserve distinct factorial identities.

10. Instrument / Measurement Tool

The operational characteristics and scoring architecture of the Checklist Individual Strength are structured as follows:

• Instrument Name: Checklist Individual Strength (CIS / CIS-20).
• Original Language: Dutch (Checklist Individuele Spankracht). Validated and widely administered in English and multiple international adaptations.
• Assessment Type: Self-administered psychological and behavioral questionnaire.
• Target Population: Adults (≥ 18 years) and older adults. Validated across healthy working adults and diverse somatic/psychiatric clinical cohorts.
• Recall Period: The preceding two weeks (“the last two weeks”).
• Total Items: 20 declarative statements.
• Authentic Response Scale: 7-point Likert scale:
  • 1 = Yes, that is true
  • 2
  • 3
  • 4
  • 5
  • 6
  • 7 = No, that is not true
• Scoring and Transformation Rules:
  • Each item is scored on a numerical scale from 1 to 7.
  • Reverse Scoring: Items are scored so that higher final numerical values represent higher levels of fatigue, impairment, or dysfunction. Because response 1 corresponds to “Yes, that is true” and 7 corresponds to “No, that is not true”, statements indicating fatigue, weakness, or impairment are reverse-scored (recoded as 1 = 7, 2 = 6, 3 = 5, 4 = 4, 5 = 3, 6 = 2, 7 = 1). Statements expressing positive energy, fitness, good concentration, and high activity retain their direct numeric values (where 7 represents the total absence of that strength, i.e., high impairment).
  • Reverse-Scored Items (per source specifications): Items 1, 3, 4, 9, 10, 13, 14, 17, 18, 19.
  • Total Score Range: 20 to 140 points (higher scores reflect greater global exhaustion and functional impairment).
• Subscale Structural Breakdown:
  • Subjective Feeling of Fatigue (8 items): Items 1, 4, 6, 9, 12, 14, 16, 20. Score range: 8 to 56 points. (Cut-off: ≥ 35 points indicates severe, clinically elevated fatigue).
  • Concentration (5 items): Items 3, 8, 11, 13, 19. Score range: 5 to 35 points.
  • Motivation (4 items): Items 2, 5, 15, 18. Score range: 4 to 28 points.
  • Physical Activity (3 items): Items 7, 10, 17. Score range: 3 to 21 points.

11. Permissions & Fee and Test Year

The Checklist Individual Strength was first formally published in 1994 by Dr. Jan H. M. M. Vercoulen and his research colleagues at the Radboud University Nijmegen Medical Centre. The copyright is retained by the original authors and academic institutions.

Permissions and Accessibility: The CIS is widely accessible for academic, non-commercial, clinical, and scientific research purposes without royalty fees. Clinical researchers and investigators wishing to utilize the instrument in formal clinical trials or institutional batteries are encouraged to cite the foundational validation papers (Vercoulen et al., 1994, 1997). Commercial organizations, pharmaceutical sponsors, or digital health platforms seeking to embed the scale within proprietary software or for-profit clinical trial registries should request formal administrative authorization through the Radboud University Expert Centre for Chronic Fatigue (ECCF).

12. References

• Bleijenberg, G., & Fennis, J. F. (1989). Anamnestic characteristics of patients with the chronic fatigue syndrome. The Netherlands Journal of Medicine, 35(3–4), 153–162.
• Bültmann, U., de Vries, M., Beurskens, A. J., Bleijenberg, G., Vercoulen, J. H., & Kant, I. J. (2000). Measurement properties and its dimensional structure of the Checklist Individual Strength (CIS) in the working population. Journal of Psychosomatic Research, 49(2), 135–142. https://doi.org/10.1016/s0022-3999(00)00155-3
• Prinsen, C. A., Lindeboom, R., Bleijenberg, G., Vercoulen, J. H., & de Vet, H. C. (2012). Psychometric properties of the Checklist Individual Strength (CIS20) in patients with neuromuscular diseases: an item response theory approach. Quality of Life Research, 21(9), 1609–1619. https://doi.org/10.1007/s11136-011-0080-z
• Servaes, P., Verhagen, S., & Bleijenberg, G. (2002). Fatigue in disease-free cancer patients: Comparing cancer survivors with healthy individuals and cancer patients with recurrence. Journal of Psychosomatic Research, 53(6), 1159–1166. https://doi.org/10.1016/s0022-3999(02)00346-2
• Vercoulen, J. H., Swanink, C. M., Fennis, J. F., Galama, J. M., van der Meer, J. W., & Bleijenberg, G. (1994). Dimensional assessment of chronic fatigue syndrome. Journal of Psychosomatic Research, 38(5), 383–392. https://doi.org/10.1016/0022-3999(94)90099-2
• Vercoulen, J. H., Alberts, M., & Bleijenberg, G. (1999). The Checklist Individual Strength (CIS). Expert Centre for Chronic Fatigue, Radboud University Nijmegen Medical Centre: Nijmegen, The Netherlands.
• Worm-Smeitink, M., Gielissen, M., Bloot, L., van Laarhoven, H. W., van Engelen, B. G., van Riel, P., Bleijenberg, G., Nikolaus, S., & Knoop, H. (2017). The assessment of fatigue: Psychometric qualities and norms for the Checklist Individual Strength. Journal of Psychosomatic Research, 98, 40–46. https://doi.org/10.1016/j.jpsychores.2017.05.007

13. Items of the Scale