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Journal of Psychosomatic Research, Vol. 39, No. 5, pp. 315-325, 1995 Copyright © 1995 Elsevier Science Ltd Pergamon Printed in Great Britain. All rights reserved 0022-3999/95 $9.50 +
   ergamon Journal of Psychosomatic Research Vol. 39, No. 5, pp. 315-325, 1995 Copyright © 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0022-3999/95 9.50 + .00 002,2-3999(94)00125-1 THE MULTIDIMENSIONAL FATIGUE INVENTORY MFI) PSYCHOMETRIC QUALITIES OF AN INSTRUMENT TO ASSESS FATIGUE E. M. A. SMETS,* B. GARSSEN,t B. BONKE:~ and J. C. J. M. DE HAES*§ Received for publication 19 October 1994) Abstract--Tile Multidimensional Fatigue Inventory (MFI) is a 20-item self-report instrument designed to measure fatigue. It covers the following dimensions: General Fatigue, Physical Fatigue, Mental Fatigue, Reduced Motivation and Reduced Activity. This new instrument was tested for its psychometric properties in cancer patients receiving radiotherapy, patients with the chronic fatigue syndrome, psychology students, medical students, army recruits and junior physicians. We determined the dimensional structure using confirmatory factor analyses (LISREL's unweighted least squares method). The hypothesized five-factor model appeared to fit the data in all samples tested (AGFIs>0.93). The instrument was found to have good internal consistency, with an average Cronbach's alpha coefficient of 0.84. Construct validity was established after comparisons between and within groups, assuming differences in fatigue based on differences in circumstances and/or activity level. Convergent validity was investigated by correlating the MFI-scales with a Visual Analogue Scale measuring fatigue (0.22<r<0.78). Results, by and large, support the validity of the MFI. INTRODUCTION Commonly the term fatigue refers to a normal everyday experience that most individuals report after inadequate sleep or rest, or after exertion of physical power. People also report feelings of fatigue after mental effort or when they lack the motivation to initiate activities. Apart from this everyday use, the term fatigue also describes a symptom considered to indicate the presence of disease Persistent fatigue is probably the symptom most frequently reported to physicians. An overview of studies on fatigue in primary care [1] showed that prevalence rates vary between 7 and 45 . This large range in prevalence rates can most likely be attributed to differences in the working definition of fatigue and to differences in measuring methods. Many physical diseases, in particular chronic diseases such as cancer, multiple sclerosis, arthritis, renal disease, and HIV infection are associated with fatigue [2-5]. * Academic Medical Centre, University of Amsterdam, Department of Medical Psychology, Amsterdam, The Netherlands. t The Helen Dowling Institute for Biopsychosocial Medicine, Rotterdam, The Netherlands. J; Erasmus University Rotterdam, Department of Medical Psychology and Psychotherapy, Rotterdam, The Netherlands. § Working Group on Medical Decision Making/Department of Clinical Oncology, University of Leiden, Leiden, The Netherlands. 315  316 E.M.A. SMETS et al Fatigue is also the key symptom of the chronic fatigue syndrome. In addition, it is generally considered an important aspect of depression. Besides being an indicator of disease, fatigue may also result from medical treatment. For example, feelings of fatigue lasting several weeks are common during post-surgery periods of convalescence [6]. Treatments for cancer such as radio- or chemotherapy, are also acknowledged to induce feelings of tiredness [2]. Finally, several medications such as analgesics, psychopharmaca and sleeping-agents may induce daytime fatigue. Fatigue has been found to negatively affect cancer patients' self-care [7] and social activities [8]. The consequences of fatigue are also reflected in its detrimental effect on patients' quality of life [9, 10]. Because of its high prevalence and increasingly acknowledged negative effect on the patient's well-being, fatigue has become an important research variable. Besides being investigated as a symptom or side-effect, it has also been studied as a precursor of disease [11], as a diagnostic criterion [12], and as an outcome-variable by which treatment is evaluated [13]. Whatever the reason may be for including fatigue as a variable, its assessment has to be reliable and valid. Instruments available to assess fatigue in patients can be divided into one- dimensional instruments and multidimensional instruments. Of the one-dimensional measures of fatigue, the Visual Analogue Scale (VAS) as for example used by Krupp et al [3] is the simplest. Pearson and Byars [14] developed a 10-item Fatigue Feeling Checklist that was used in studies on the effects of cancer treatment [I 5, 16]. Examples of other one-dimensional fatigue questionnaires are the Rand Index of Vitality [17], the Tiredness Scale [18] and the Fatigue Severity Scale [19]. Examples of more comprehensive instruments that include a fatigue subscale are the EORTC-Quality of Life Questionnaire [10] and the Profile of Mood States (POMS) [13]. Multidimensional measures include a two-dimensional fatigue-scale as used by Wessely and Powell [20] in their study on the chronic fatigue syndrome. The questionnaire contains a physical fatigue and a mental fatigue scale. The Fatigue Symptom Checklist (FSCL) is a multidimensional questionnaire which was used by Haylock and Hart [15] and by Kobashi and co-workers [21] in cancer patients. The srcinal FSCL contains 30 symptoms and was divided into three subscales, based on a factor analysis: (1) general feelings of sleepiness, with items like feel tired in the legs and want to lie down ; (2) mental feelings of fatigue, with items like difficulty in thinking and become nervous ; and (3) specific bodily sensations, such as headache and dizziness [22]. Piper and colleagues developed an instrument to measure the experience of fatigue in patients, which they termed the Piper Fatigue Self-report Scale (PFS) [23]. This instrument consists of 41 visual analogue scales representing the temporal, intensity, affective and sensory dimensions of fatigue. Results were obtained in a sample of breast and lung cancer patients. Whether the assumed dimensions of fatigue were reflected in the actual data of the patient population was not mentioned. Many patients had difficulties completing the questionnaire. The authors, however, report excellent reliability and moderate construct validity of the instrument. The last questionnaire to be discussed, is the 48-item Dutch questionnaire, developed by Vertommen and Leyssen [12]. The instrument was tested on a student- and patient-population, yielding a three-factor solution: general fatigue, mental fatigue and somatic symptoms, for both groups.  The Multidimensional Fatigue Inventory 317 One-dimensional measures of fatigue are frequently used in studies including patient samples. One can, however, question their adequacy. Individuals with the same overall fatigue score may differ in their experiences. One person might, for instance, feel physically exhausted and mentally alert, while a second one feels mentally tired but physically fit. The use of one-dimensional instruments excludes this possibility of a complete description of the fatigue experience of patients. The available, more comprehensive multi-dimensional instruments have two major drawbacks. They are~-except for the instrument developed by Wessely and Powell either lengthy, which might hamper completion in patient groups for whom fatigue is a key symptom, or they contain other somatic symptoms beside those referring directly to tiredness, such as headache in the FSCL. This induces the risk of contamination of fatigue with somatic illness. Prompted by the lack of an instrument meeting these problems, we decided to develop (1) a short questionnaire that, (2) would not contain any somatic items, and (3) had to be multidimensional. The present paper describes the development of this questionnaire and the research investigating its dimensional structure, reliability and validity. METHOD The questionnaire At the onset of developing the questionnaire five dimensions of fatigue were postulated based on the manners in which fatigue can be expressed. Firstly, fatigue can be expressed by general remarks of a person concerning his or her functioning, for example I feel rested . Secondly, by referring to physical sensations, related to the feeling of tiredness. Thirdly, by referring to cognitive symptoms, such as having difficulty concentrating. These three dimensions, labelled General, Physical and Mental Fatigue correspond with scales formed by factor-analyses as reported by others [12~1]. The term fatigue is also used to describe a lack of motivation to start any activity [24]. This resulted in the fourth dimension, labelled Reduced Motivation. The final dimension refers to a frequently occurring, although not necessary consequence of fatigue, namely a reduction in activity, and is labelled Reduced Activity. We termed the resulting instrument the Multidimensional Fatigue Inventory (MFI). In constructing the questionnaire, an equal number of items for each of the five postulated dimensions was strived for. Items were worded in a positive and a negative direction to prevent tendencies towards the response set. For each dimension the number of positively, and respectively negatively formulated items should be well balanced and redundancy of the number of items had to be precluded: the more parsimonious the questionnaire the better. However, because of difficulties in formulating equally acceptable positively and negatively framed items, it was decided to leave the test-version out of balance. Items were later removed based on the results obtained. Ultimately, results are presented in this paper on the MFI consisting of 24 statements for which the person has to indicate on a 7-point scale to what extent the particular statement applies to him or her (Fig. 1). The statements refer to aspects of fatigue experienced during the previous days. Higher scores indicate a higher degree of fatigue. rocedure To test the MF1 with patients, data were gathered from a heterogeneous group of cancer patients treated with radiotherapy and from individuals who participated in a study investigating the chronic fatigue syndrome. First year psychology students, and medical students, completed the questionnaire to investigate the properties of the instrument when used with healthy persons under normal circumstances. In addition, the MFI was filled in twice by junior physicians, before and after 5-6 wk of their first practical training in internal medicine. This sample was included to investigate the MFI when used in conditions assumed to induce fatigue. Typical for this medical training period is the large number of new impressions, late hours and high emotional burden. We also wanted a sample for which we assumed that fatigue was primarily the result of physical effort. Therefore, data were obtained from two groups of army recruits.  318 E. M. A. SMETS et al ,, l I I I I I I 2. Physi.,y,f lonlyabletodoa,itt,0 ,°s. th is'--I I I I I I I 3.,f e, voryaotivo I I I I I I no,°°'th i' no that is 4. I am not up to much yes thatistruel I I ] I I [ not true 5. Thinking requires effort ycs that s true [ [ ] I I I not o that is rue 1. Investigators interested in using the instrument should contact the first author. An authorised English version is available. For academic use permission will be granted at no charge but while still under development investigators will be requested to share their results with the authors so that reliability and validity testing can proceed appropriately. These soldiers were assessed either during their stay in the barracks, or in the second week of a physically intensive military training program. To investigate whether the assumption of five dimensions was supported by the data, confirmatory factor analyses were conducted. An advantage of confirmatory over exploratory factor analysis is that one can specify a priori the kind of interpretation one prefers. In an exploratory factor model the researcher does not specify the structure of the relationships among the variables in the model. In the confirmatory factor model, however, the researcher poses constraints, preferably motivated by theory, determining the relation between variables, common factors and unique factors. The instrument must be shown to measure the same construct in different groups to compare results from one sub-group to another. The factor-structure was therefore investigated for the various study-groups, with the exception of the junior physicians. (For this group only validity data will be presented.) Next, internal consistency was assessed for all scales. Assessing the construct validity of the MFI had high priority in this investigation. Construct validity is generally determined by testing hypotheses concerning the construct to be measured. First, assessment of the construct validity of the MFI was based on comparisons between groups that supposedly differ in fatigue. Patients with CFS and radiotherapy patients were assumed to be more fatigued than the students and the army recruits in the barrack situation. It was also assumed that junior physicians during training and army recruits during exercise would be more fatigued than the students and the soldiers in the barrack situation. Secondly, construct validity was determined based on hypothesized differences in fatigue within a sample, resulting from differences in activity. For this purpose, psychology students were asked to indicate whether they had done any exceptional activities in the days before completing the questionnaire. Students who for example reported that they had gone out a good deal were expected to have higher scores than students who did not report any exceptional activities during the previous days. In addition, an increase in fatigue scores was expected in junior physicians between the two assessments. Convergent validity is an indicator of the degree to which a newly developed instrument is related to already existing instruments intended to measure the same construct under investigation, In order to investigate convergent validity of the MFI, radiotherapy patients were asked to indicate the intensity of their fatigue as experienced during the previous days on a 100 mm Visual Analogue Scale (VAS), ranging from not at all tired too extremely tired . Subjecl A heterogeneous group of cancer patients (n = 111) completed the questionnaire, at the out-patient clinic immediately after receiving radiation, using a cross-sectional design. One-hundred and thirteen patients were asked to participate. Two patients refused because they felt too tired. The average age of this sample was 61 yr. Of the respondents, 59 were male and 52 female. The sample of chronic fatigued patients consisted of patients who experienced severe, disabling fatigue, of definite onset, lasting for more than 12 months. Three hundred and ninety-five self-referred patients were sent a postal questionnaire.
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