Spanish version of the Swedish Occupational Fatigue Inventory (SOFI): Factorial replication, reliability and validity

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International Journal of Industrial Ergonomics 35 (2005) 737–746 www.elsevier.com/locate/ergon

Spanish version of the Swedish Occupational Fatigue Inventory (SOFI): Factorial replication, reliability and validity Jose´ Luis Gonza´lez Gutie´rreza,, Bernardo Moreno Jimeńezb, Eva Garrosa Hernańdezb, Almudena Lo´pez Lo´peza a

Universidad Rey Juan Carlos, Facultad de Ciencias de la Salud, Avda. Atenas s/n, 28922 Alcorcoń (Madrid), Spain b Universidad Autońoma de Madrid, Facultad de Psicologıá, Ctra. de Colmenar km. 15, 28049 Madrid, Spain Received 10 April 2004; received in revised form 25 September 2004; accepted 2 February 2005 Available online 15 April 2005

Abstract This paper presents the adaptation to Spanish of the Swedish Occupational Fatigue Inventory (SOFI) (Ahsberg, Gamberale and Kjellberg, 1997), an instrument for the multidimensional evaluation of work-related fatigue. A total of 240 nurses working in eight special attention units responded to a pool of 25 items about their level of fatigue after work. Proposed SOFI structure (Ahsberg, 2000) was tested by means of Confirmatory Factor Analysis, and the data demonstrate an acceptable fit to the theoretical five-factor model (with lack of energy defined as a general latent factor representing much of the variance of the items) when the number of items was reduced to 15. Alpha coefficients were calculated, and high internal consistency values were obtained for most of the subscales. Convergence was also evaluated by calculating correlations between the SOFI subscales and a number of independent indices. All five resulting subscales make up a promising 15-item instrument for the evaluation of work-related fatigue in the Spanish language. Relevance to industry: A reliable and valid instrument for the multidimensional evaluation of work-related fatigue may be of great importance to an understanding of the origin and development of work-related disorders. This study presents the adaptation to Spanish of the SOFI, a highly promising instrument, given the lack of multidimensional instruments adapted to the Spanish language for the measurement of work-related fatigue. r 2005 Elsevier B.V. All rights reserved. Keywords: Perceived fatigue; Swedish occupational inventory; Validity; Reliability; Factorial replication; Questionnaire

1. Introduction Corresponding author. Fax: +34 91 4888831.

E-mail address: [email protected] (J.L. Gonza´lez Gutie´rrez).

Fatigue has been identified as one of the commonest problems in developed countries (Lewis and Wessely, 1992), and a serious threat

0169-8141/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.ergon.2005.02.007

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to quality of life when it becomes chronic or excessive (Piper, 1989). In spite of this, the concept is ill-defined, fundamentally due to the inclination to refer in the same linguistic terms to the presence of different conditions (Ahsberg, 1998). In some cases, this lack of definition has led authors to exclude the phenomenon from scientific discussion (Munscio, 1921). Nevertheless, it is more frequently considered as a multidimensional construct that differs among different levels of response. Bills’ (1934) approach has been the most widely accepted in this respect. He drew a distinction between three aspects of fatigue: physiological fatigue (reduction of physical capacity), objective fatigue (reduction in work) and subjective fatigue (feelings of weariness). Following the proposal of this author, most of the operative definitions have been grouped in bodily changes (Bigland-Ritchie, 1984; Christensen, 1962; Eidelman, 1980), changes in performance (Bartlett, 1953; Browne, 1953; Hemingway, 1953; Mital et al., 1994; Welford, 1953) and perceptual changes (Britton, 1983; Piper, 1986; Wessely et al., 1998). Each of these three levels of response in fatigue is associated with a specific method of evaluation. Physiological parameters, such as muscular activity (EMG) (Hagberg, 1981; Ha¨gg et al., 1987; Malmqvist et al., 1981), blood pressure and heart rate (Bystrom et al., 1991; Kilbom et al., 1983), oxygen consumption (Gamberale, 1972), cerebral activity (Torsvall and Akerstedt, 1987), melatonin level (Akerstedt et al., 1982), urinary cortisol (Melamed and Bruhis, 1996), and changes in critical flicker fusion frequency (Rey and Meyer, 1980) have been used as indicators of physiological manifestations of fatigue. Behavioural manifestations, mainly in the form of deterioration in performance (Bartlett, 1943), have been studied, for example, through the calculation of reaction times (Kjellberg et al., 1996), or error rate during tasks (Henning et al., 1989). Finally, perceived fatigue has been assessed by uni-dimensional scales, including a single question about how tired the person feels (Goldmark, 1912; Lee et al., 1991; Monk and Folkard, 1985; O¨berg et al., 1994; Okogbaa et al., 1994; Rabinach, 1992;

Rimehaug and Svebak, 1987; Schaeffer et al., 1995). Within this perspective, Borg has developed several rating scales that have been correlated with physiological responses and physical load, among them his successful Rating of Perceived Exertion (RPE; Borg, 1970, 1998) and CR-10 (Borg, 1982, 1998). Another interesting instrument is the Fatigue Severity Scale (FSS; Lichstein et al., 1997), which provides an exclusive score on severity of fatigue by means of nine items. These instruments allow researchers to obtain an estimation of the intensity of perceived fatigue, but do not offer information about its quality. As Ahsberg (1998) points out, even if the same intensity of fatigue can be perceived after different work tasks, the perception of fatigue may be of a different nature. A multidimensional approach to fatigue allows the simultaneous evaluation of intensity and quality of perceived fatigue (Kinsman and Weiser, 1976). Prominent among the instruments developed from this perspective is the Multidimensional Fatigue Inventory (MFI; Smets et al., 1995, 1996), designed for use within the clinical context and employed in the evaluation of fatigue in cancer patients. The validity of this questionnaire was examined through confirmatory factorial analysis, and the five factors were interpreted as General fatigue, Physical fatigue, Mental fatigue, Reduced motivation and Reduced activity. Another instrument of great interest for clinical practice is the Piper Fatigue Self-Report Scale (Piper et al., 1989), initially developed for the evaluation of patients exposed to radiotherapy treatment. This instrument proposes seven dimensions of fatigue: Temporal, Intensity, Affective, Sensory, Evaluative, Associated symptoms and Relief. Finally, the FACES questionnaire (Shapiro et al., 2002), allows the evaluation of five dimensions of fatigue: Fatigue, Energy, Consciousness, Energized and Sleepiness. Reliability and validity of the instrument, for a sample of 372 patients with sleep disorders, were satisfactory. Despite the above-mentioned studies, and with some exceptions, no systematic attempt has been made to develop self-report scales for the evaluation of perceived fatigue in occupational settings. As Ahsberg (1998) points out, it is not clear to what extent fatigue characteristics are, as factors

ARTICLE IN PRESS J.L. Gonza´lez Gutie´rrez et al. / International Journal of Industrial Ergonomics 35 (2005) 737–746

associated with the process of disease, correspond to the factors present in the process of workrelated fatigue. The characteristics of fatigue change depending on the specific contexts, and several studies have been carried out with the aim of identifying meaningful dimensions involved in the perception of work-related fatigue. The qualitative aspects of fatigue most frequently described have been physical fatigue and mental fatigue (Chalder et al., 1993; Grandjean, 1979), while other aspects of the construct usually studied are sleepiness (Gillberg et al., 1994) and discomfort (Cameron, 1996). A study among railroad workers identified two dimensions of fatigue: Weakened activation and Weakened motivation (Kashiwagi, 1969). On the other hand, Matthews and Desmond (1998) identified four basic dimensions of fatigue as result of exposure to a simulated driving task: Boredom, Visual fatigue, Malaise and Muscular fatigue, while Saito and Kashiwagi (1970) extracted the following dimensions from a pool of 30 fatigue symptoms in a sample of 9575 industrial workers: Drowsiness and dullness, Mental symptoms, and Projection of physical disintegration. These dimensions were used later in a study with 17,789 participants, in which the first dimension (Drowsiness and dullness) proved to be common to a great variety of professions, whereas the second (Mental symptoms) and third (Projection of physical disintegration) were found to be present in mental and physical activities, respectively (Yoshitake, 1978). Ahsberg et al. (1997) have developed the Swedish Occupational Fatigue Inventory (SOFI), a multidimensional instrument for measuring fatigue based on self-reports. This questionnaire

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proposes five dimensions extracted from 95 verbal expressions describing feelings of fatigue that were rated by 705 workers from 14 different professions (including two groups of students). These dimensions were labelled as Lack of energy, Physical exertion, Physical discomfort, Lack of motivation and Sleepiness (see Table 1). According to the authors, the underlying structure of the instrument corresponds to a new qualitative and quantitative description of the physical (physical exertion and physical discomfort) and mental (lack of motivation and sleepiness) dimensions of perceived fatigue. Together with these, the factor ‘‘lack of energy’’ corresponds to a fatigue dimension with both physical and mental characteristics (Ahsberg, 1998). Originally, the SOFI (Ahsberg et al., 1997) was made up of 25 expressions (five for each dimension) related to physiological, cognitive, motor and emotional responses. Depending on the research intentions, participants were asked to rate on an 11-point scale the extent to which the expressions were describing their own feelings at that moment, during the last few minutes, when they were most tired, and so on. The instrument was subsequently revised by Ahsberg (2000) using confirmatory factorial analysis, and the number of expressions in each dimension was reduced to four (the questionnaire finally consisted of 20 elements). Previously, in accordance with the information from earlier research, two of the original expressions had been replaced by new ones. Finally, the response scale was changed to one with seven points. The factorial validity of this new version was better than that of the previous model (Ahsberg,

Table 1 Description of the fatigue dimensions proposed by the Swedish Occupational Fatigue Inventory (SOFI; Ahsberg et al., 1997) Dimension

Description

Lack of energy Physical exertion

This dimension describes general feelings of diminished strength This dimension describes whole-body sensations that may be the result of dynamic work and, to a certain extent, the sign of metabolic exhaustion This dimension describes more local bodily sensations that may result from static or isometric workload This dimension describes feelings of not being involved or enthusiastic This dimension describes feelings of sleepiness

Physical discomfort Lack of motivation Sleepiness



2000). Concurrent and discriminant validity were assessed by comparing the scores obtained by different types of worker (teachers, firemen, cashiers, bus drivers and engineers) on each dimension. Internal consistency of the subscales was satisfactory, with Cronbach’s alphas of over .80, especially for Lack of energy (.92), Lack of motivation (.92) and Sleepiness (.89). Slightly smaller values were obtained for Physical discomfort (.81) and Physical exertion (.87). The present work proposes to examine one of the ‘‘forgotten’’ areas within the Spanish-speaking context, given the lack of multidimensional instruments adapted to the Spanish language for the measurement of work-related fatigue. SOFI has been chosen because of the strength of its theoretical structure, shown in a wide range of studies focusing on the improvement of the instrument’s psychometric properties.

Table 2 Distribution for the pool of 25 items initially proposed for the adaptation process Dimension

Lack of energy

Proposed items English

Spanish

Overworked

Habiendo trabajado en exceso Agotado Exhausto Desgastado Extenuado

Worn out Exhausted Spent Drained Physical exertion

Sweaty Breathing heavily Palpitations Warm Out of breath

Physical discomfort

Tense muscles

Aching

Con los mu´sculos en tensioń Con las articulaciones agarrotadas Entumecido Lastimado muscularmente Dolorido

Lack of motivation

Uninterested Passive Listless Indifferent Lack of involvement

Falto de intere´s Pasivo Apa´tico Indiferente Falto de implicacioń

Sleepiness

Sleepy Falling asleep Drowsy Yawning Lazy

Somnoliento Durmieńdome Amodorrado Bostezante Con pereza

Stiff joints

2. Method Numbness Hurting

2.1. Sample Two hundred and forty full-time nurses working in eight special attention units participated in the adaptation of the instrument. Thirty individuals were taken from each of these units, representing 12.5% of the sample. The average age was 35.41 years (7SD 8.10), and the average experience in the profession was 13.28 years (7SD ¼ 7.85); 86.7% of the participants were women and 13.3% were men. Average duration of continuous work was 9.86 (7SD 6.31). Shift distribution was rotating shift (42%), morning shift (20%), evening shift (18.75%), night shift (8.75%) and extended 24-h shift (10.42%). 2.2. Instruments The adaptation process began with a review of the 25 items from the initial version of the SOFI (Ahsberg et al., 1997). One of the original expressions (‘‘taste of blood’’) had previously been replaced (by ‘‘warm’’), following the authors’ recommendations (Ahsberg and Gamberale, 1998) (see Table 2). In addition, ‘‘lack of concern’’

Sudoroso Respirando con dificultad Con palpitaciones Con calor Sin aliento

was replaced by ‘‘lack of involvement’’, due to its similarity with other expressions making up the Lack of motivation scale (once translated into Spanish). These 25 items, representing subjective feelings of weariness, were translated into Spanish by means of translation and back-translation (Brislin, 1986). The pool of translated items was rated by the 240 nurses using an 11-point response scale. Following the indications of the authors, who pointed out the need for participants’ ratings


to be related to a particular moment in time or a specific period, interviewees were asked to indicate their typical feelings at the end of the working day. As a step in testing the validity of the SOFI adaptation, the NASA-TLX Effort and Frustration subscales (Hart and Staveland, 1988) were administered without the usual weighting phase, these having been satisfactorily used by authors such as Nygren (1991), Hendy et al. (1993) or Moroney et al. (1995). NASA-TLX is a multidimensional self-reported assessment technique that provides an estimation of the overall workload associated with task performance. The assessment is comprised of the relative contribution of six underlying psychological factors to overall workload: mental demand, physical demand, temporal demand, performance, effort (mental and physical) and frustration level. NASA-TLX has been found to be the most valid measure of subjective workload, to have the highest user acceptance, and to have the smallest between-subject variability (Vidulich and Tsang, 1986; Byers et al., 1988; Hart and Wickens, 1990; Hill et al., 1992). NASA-TLX ratings have also been shown to be sensitive to experimentally manipulated levels of workload and to be more reliable than other subjective techniques (Aretz et al., 1996). Finally, in addition to the application of the NASA-TLX Effort and Frustration subscales, information on the usual duration of participants’ shift was recorded. 2.3. Statistical treatment of data To test the five-factor structure of fatigue proposed by Ahsberg (2000) in the revision of

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SOFI, confirmatory factor analyses were performed by means of the Structural Equation Modelling (Crowly and Xitao, 1997; Mueller, 1996). The covariance matrix was analysed using the maximum likelihood estimation method (Hoyle, 1995). Internal consistency of the subscales was evaluated by calculating their alpha coefficients. Finally, in order to analyze the convergent validity of the instrument, intercorrelations between the SOFI subscales and the NASATLX Frustration and Effort subscales were calculated, as well as between the SOFI subscales and normal shift duration.

3. Results As a start, two Confirmatory Factor Analyses were preformed with the purpose of testing the five-factor structure of fatigue proposed by Ahsberg (2000) in the revision of SOFI. Initially, a simple one-factor where the 25 variables (items) were related to one latent factor was confirmatively tested. This model assumes that fatigue is a coherent concept consisting of only one dimension, and the results showed a very unsatisfactory fit to the data (Table 3). Then, the hypothesized model (25 observed variables and four latent variables plus an additional general latent variable) was tested. The results showed that this theoretical model was significantly better as compared to the one-factor model, but still unsatisfactory. GFI was lower than .90, AGFI was less than .80 and RMSEA is over .08, indicating a need to re-specify the model (v.g.

Table 3 Swedish occupational Fatigue Inventory (SOFI): Summary of maximum likelihood confirmatory factor analysis results. The hypothesized model is compared to the one-factor model, the revised model with 20 manifest variables is compared to the hypothesized model, and the revised model with 15 manifest variables is compared to the revised model with 20 manifest variables Model

w2 /df

diff w2 /df

Dw2 /Ddf

RMSEA

p

GFI

AGFI

One-factor, 25 var. Hypothesized, 25 var. Revised, 20 var. Revised, 15 var.

2146/275 861/249 495/148 205/72

— 1285/26 366/101 290/76

— 49.42 3.62 3.81

.17 .10 .10 .08

.00 .00 .00 .00

.48 .77 .83 .90

.38 .70 .75 .83

Note: w2 /df ¼ relative chi-square, RMSEA ¼ root mean square error of approximation, GFI ¼ goodness of fit index, AGFI ¼ adjusted goodness of fit index.



Chin, 1998; Hair et al., 1995; Segars and Grover, 1993). Consequently, internal correlations, factor loadings, and t-values for each manifest variable were analysed and a revised model was formulated, including 20 manifest variables distributed in the same way as in the previous case. Again, results showed an unsatisfactory adjustment to the data. In consequence, a new revised model with only 15 items was formulated (again, distributed in four latent variables plus an additional general latent variable). The results indicated an acceptable, even if not wholly satisfactory, fit to the data. Relative chi-square (w2 /df), w2 -difference (Dw2 /Ddf), RMSEA, GFI and AGFI values, revealed that the 15-variables model was significantly better than the three previous models

0.65

Worn out

0.92

Exhausted

0.77

Drained α 0.82

(Table 3). Fig. 1 represents the factor solution with 15 items distributed in four latent variables plus an additional general latent variable. This final solution corresponds with the model proposed by Ahsberg (2000): a general dimension (Lack of energy) and some specific dimensions of fatigue (Physical exertion, Physical discomfort, Lack of motivation and Sleepiness). Correlations between all the factors were moderate, except for that between Physical exertion and Physical discomfort, which was slightly larger (.80). A close examination of the analysis results showed that all the items loaded in their expected target factors. However, the item ‘‘Indifferent’’ loaded minimally (.08) on the expected Lack of energy factor, and the item ‘‘Warm’’

0.29 Breathing heavily 0.63

Lack of energy

0.24

Palpitations

0.55

0.27

Warm α 0.55

0.32

0.49

Stiff joints

0.67

0.45

Numbness

0.63

0.54

Aching α 0.80

0.42

0.25

Listless

0.69

0.19

Passive

0.89

0.08

Indifferent α 0.81

0.68

0.31

Sleepy

0.92

0.80

0.17

Lack of motivation

0.22

0.55

0.78

0.36

0.80

Factor loadings

0.44 Physical discomfort 0.47

0.24 Falling asleep Yawning α 0.91

Physical exertion

Factor loadings

Sleepiness

Correlations

Fig. 1. Swedish Occupational Fatigue Inventory (Spanish version): confirmatory factor analysis with 15 manifest variables. Factor loadings, correlations between factors, and the reliability of each factor (Cronbach alpha, a), are shown.


yielded a moderate loading on Physical exertion (.32). Consequently, Cronbach alpha for Physical exertion was low (a ¼ :55). According to the result of the factor analysis, the five factors were interpreted as subscales, and in order to obtain a simple measure of each subscale, the means of the ratings were calculated for each subject. Table 4 presents means, standard deviations and kurtosis indexes of each subscale of the SOFI. Convergent validity of the SOFI was assessed by calculating the Pearson correlations between subscale scores and independent scores for related constructs (NASA-TLX subscales of Effort and Frustration, and shift duration). As Table 5 shows, in most of the cases the SOFI subscales correlated significantly with the proposed constructs. Furthermore, certain patterns of correlations supporting the differentiation among subscales were observed. For example, Effort (NASA-TLX) Table 4 Descriptive statistics: means, standard deviations and kurtosis indexes.

Lack of energy Physical exertion Physical discomfort Lack of motivation Sleepiness

Mean

SD

Min

Max

Kurt.

5.00 2.52 3.37 2.26 2.82

2.42 1.77 2.40 2.04 2.65

.00 .00 .00 .25 .00

10.00 8.33 9.33 8.33 10.00

.82 .19 .58 .05 .10

Table 5 Convergent validity of SOFI subscales. Pearson correlations between SOFI subscales, NASA-TLX subscales of Effort and Frustration, and shift duration Subscales

Effort Frustration Shift duration (NASA-TLX) (NASA-TLX)

Lack of energy Physical exertion Physical discomfort Lack of motivation Sleepiness

.349**

.302**

.178**

.250**

.155*

.043

.258**

.295**

.120

.102

.303**

.152*

.169**

.256**

.504**

*po0:05; **po0:01:

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showed a special relationship with the physical aspects of fatigue (Physical exertion and Physical discomfort), as well as with Lack of energy. On the other hand, Frustration (NASA-TLX) was especially correlated with Lack of motivation. Finally, as expected, Shift duration was particularly associated to Sleepiness.

4. Discussion The present results offer encouraging preliminary support for the reliability and validity of the Spanish version of the SOFI. The data in this study demonstrate an acceptable fit to the proposed theoretical five-factor model with Lack of energy defined as a general latent factor representing much of the variance of the items (Ahsberg, 2000). Neverthless, fit is not wholly satisfactory, as in the work of Ahsberg (2000) occurred. This was interpreted by the author appealing to the lesser number of occupations in her sample, as compared with the sample employed in her first study (Ahsberg et al., 1997). In the same way, the present analysis was based on a sample consisting exclusively of nurses. In this regard, the use of a sample made up of workers from the same occupation (with similar demands) may hinder the fit to the model. Besides, the number of items is reduced to 15, with three items in each factor. In spite of this, alpha coefficients indicated the presence of high levels of reliability for the subscales, exceeding the minimum criterion (a ¼ :502:70) recommended for research instruments (Anastasi, 1988), except in the case of Physical exertion (a ¼ :55). The low alpha value for Physical exertion is attributable to the problematic fit of the item ‘‘Warm’’ on this dimension. With a view to testing the fit of the model in a sample composed by a greater number of occupations, it would seem essential to develop further studies using heterogeneous Spanish samples with different demands (Ahsberg, 2000). In addition, these new studies should try to increase the reliability of the Physical exertion by means of the addition of new items and examining the possible suppression of the item ‘‘Warm’’. In turn, increases in the number of items per subscale,



avoiding the loss of the practical implications of limited number of items in the questionnaire, should improve the content validity of the subscales. Finally, on examining convergent validity, positive results were also yielded, since almost all the subscales were positively associated with the level of perceived physical and mental effort, as well as with the level of frustration generated by the task, measured by means of the NASA-TLX subscales. Effort (NASA-TLX) showed a special relationship with the physical aspects of fatigue (Physical exertion and Physical discomfort), as well as with Lack of energy. Frustration (NASA-TLX) was especially correlated with Lack of motivation. Similarly, as was expected, except Physical discomfort and Physical exertion, the remaining subscales were related to the usual duration of shift, and especially Sleepiness. In conclusion, the resulting adaptation to Spanish includes 15 expressions related to physiological, cognitive, motor and emotional responses, through which five basic fatigue dimensions (Lack of energy, Lack of motivation, Sleepiness and Physical discomfort and Physical exertion) can be measured. Reliability of the instrument was satisfactory, and examination of convergence also yielded encouraging results. Nonetheless, it seems necessary to undertake new studies based on more heterogeneous samples (in relation to work demands), in order to test the fit of the model in a sample composed by a greater number of occupations, as well as with the aim of improve the reliability of the Physical exertion subscale. Gathering new data on the instrument’s reliability and convergent and divergent validity seems also necessary. In any case, this adaptation constitutes a highly relevant product, bearing in mind the previous lack of adequate tools for measuring work-related fatigue in the Spanish language context.

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