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209
Psychiatry Research, 46:20!9-210 Elsevier
The Author
Replies
To the Editors: I wish to thank Dr. Chambers and Ms. Docktor for their critical review and the opportunity for dialogue that will enable this area of research to move forward. They question the validity and reliability of the visual analogue scale for fatigue (VAS-F) given our failure to distinguish between fatigue and sleepiness and our choice of sampling strategies. Very few measures for estimating fatigue severity are available. For instance, the Fatigue Severity Scale (Krupp et al., 1989) measures the impact of fatigue rather than its severity. As Chambers and Docktor are aware, the “gold standard” for assessing sleepiness is the multiple sleep onset latency test (MSLT) using polysomnographic measures (Stepanski et al., 1988). Others have estimated sleepiness by frequency of “nodding off” or more more sophisticated measures of “microsleeps” in sedentary situations (Rosekind et al., 1991). Unfortunately, there is no “gold standard” for estimating subjective fatigue. To establish content validity, researchers often use expert panels or content analysis of interviews about particular phenomena so that an instrument will measure what it is intended to measure. The VAS-F was developed from 37 adjectives used in the literature and from people complaining of fatigue. Principal components analysis with eigenvalues > 1 was used to identify five components, and the 18 items in the final VAS-F loaded on the same two factors.
0165-1781/93/$06.00
@ 1993 Elsevier Scientific
When the loadings were restricted to three factors, these items loaded on the same factor (eigenvectors > 0.75). The five items related to feeling energetic, even if reverse-scored, loaded on a second factor and therefore could not be assumed to be, as Chambers and Docktor argue, “simply two sides of the same coin.” The 15 eliminated items loaded equally on three or more factors on the basis of either unrotated component loadings or varimax rotations, were related to anxiety, or were culturally biased terms. It was not possible to distinguish sleepiness from fatigue by a variety of principal component analyses in our heterogeneous sample of healthy persons or in our sample of sleep disorder patients, and to do so now with the 18-item VAS-F would go beyond the data. To establish criterion or predictive validity (Carmines and Zeller, 1978) the VAS-F and its potential subscales for sleepiness and fatigue need to be tested further in a variety of samples. Perhaps, then, the distinction between sleepiness and fatigue can be elucidated. In the healthy group, fatigue scores were lower in the morning compared with their evening scores. These two factors help to establish validity and reliability of the VASF. Because perception of any symptom changes over time, test-retest reliability is not appropriate, particularly in this case when there are known circadian fluctuations and environmental influences on fatigue, energy levels, alertness, and sleepiness.
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Ireland
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210 I also find, as Chambers and Docktor stated, that the Stanford Sleepiness Scale (SSS) is fast and easy to administer. However, it is often difficult for subjects to select only one option within the SSS because the adjectives within each statement are not mutually exclusive from adjectives in the other seven options. When subjects select more than one option, their results must be eliminated from data analysis. Furthermore, with a categorical 7-point scale, variance is limited. The psychometric properties of the SSS should be examined and weighed against its ease of administration. If readers refer to Tables 4 and 5 (Lee et al., 1991), there are interesting data about the SSS: healthy subjects were more sleepy in the evening (mean = 4.0, SD = 1.59 on the SSS) than sleep disorder patients (mean = 3.0, SD = 1.64). In the morning, the mean SSS scores decreased to 2.4 (SD = 1.36) for healthy subjects and 2.7 (SD = 1.30) for patients. For research purposes, these small effect sizes require larger samples to detect statistically significant differences. Chambers and Docktor argue that distinguishing between fatigue and sleepiness is clinically important. That distinction is also important for researchers. Unfortunately, individuals may not be able to distinguish between sleepiness and fatigue with the precision that researchers and clinicians would like. Estimating any subjective state has its limitations. It is only through dialogue such as this that measurement issues become refined. Perhaps, with new samples and replication of studies, subscales specifically delin-
eating sleepiness from fatigue will emerge from the VAS-F. References Carmines, E.G., and Zeller, R.A. Reliability and Validity Assessment. Sage University Paper series on Quantitative Applications in the Social Sciences, series no. 07-017. Beverly Hills and London: Sage Publications, Inc., 1978. Krupp, L.B.; LaRocca, N.G.; Muir-Mash, J.; and Steinberg, A.D. The Fatigue Severity Scale. Archives of Neurology, 46: 112I- 1123, 1989. Lee, K.A.; Hicks, G.; and Nino-Murcia, G. Validity and reliability of a scale to assess fatigue. Psychiatry Research, 36:291-298, 1991. Rosekind, M.R.; Connell, L.J.; Dinges, D.F.; Rountree, M.S.; and Graeber, R.C. Preplanned cockpit rest: EEG sleep and effects on physiological alertness. Sleep Research, 20: 129, 199 1. Stepanski, E.; Zorick, F.; Roehrs, T.; Young, D.; and Roth, T. Daytime alertness in patients with chronic insomnia compared with asymptomatic control subjects. Sleep, 11:54-60, 1988. Kathryn A. Lee, R.N., Ph.D. Department of Family Health Care Nursing School of Nursing University of California, San Francisco San Francisco, CA, USA December IS. 1992
Psychiatry Research, 46:20!9-210 Elsevier
The Author
Replies
To the Editors: I wish to thank Dr. Chambers and Ms. Docktor for their critical review and the opportunity for dialogue that will enable this area of research to move forward. They question the validity and reliability of the visual analogue scale for fatigue (VAS-F) given our failure to distinguish between fatigue and sleepiness and our choice of sampling strategies. Very few measures for estimating fatigue severity are available. For instance, the Fatigue Severity Scale (Krupp et al., 1989) measures the impact of fatigue rather than its severity. As Chambers and Docktor are aware, the “gold standard” for assessing sleepiness is the multiple sleep onset latency test (MSLT) using polysomnographic measures (Stepanski et al., 1988). Others have estimated sleepiness by frequency of “nodding off” or more more sophisticated measures of “microsleeps” in sedentary situations (Rosekind et al., 1991). Unfortunately, there is no “gold standard” for estimating subjective fatigue. To establish content validity, researchers often use expert panels or content analysis of interviews about particular phenomena so that an instrument will measure what it is intended to measure. The VAS-F was developed from 37 adjectives used in the literature and from people complaining of fatigue. Principal components analysis with eigenvalues > 1 was used to identify five components, and the 18 items in the final VAS-F loaded on the same two factors.
0165-1781/93/$06.00
@ 1993 Elsevier Scientific
When the loadings were restricted to three factors, these items loaded on the same factor (eigenvectors > 0.75). The five items related to feeling energetic, even if reverse-scored, loaded on a second factor and therefore could not be assumed to be, as Chambers and Docktor argue, “simply two sides of the same coin.” The 15 eliminated items loaded equally on three or more factors on the basis of either unrotated component loadings or varimax rotations, were related to anxiety, or were culturally biased terms. It was not possible to distinguish sleepiness from fatigue by a variety of principal component analyses in our heterogeneous sample of healthy persons or in our sample of sleep disorder patients, and to do so now with the 18-item VAS-F would go beyond the data. To establish criterion or predictive validity (Carmines and Zeller, 1978) the VAS-F and its potential subscales for sleepiness and fatigue need to be tested further in a variety of samples. Perhaps, then, the distinction between sleepiness and fatigue can be elucidated. In the healthy group, fatigue scores were lower in the morning compared with their evening scores. These two factors help to establish validity and reliability of the VASF. Because perception of any symptom changes over time, test-retest reliability is not appropriate, particularly in this case when there are known circadian fluctuations and environmental influences on fatigue, energy levels, alertness, and sleepiness.
Publishers
Ireland
Ltd.
210 I also find, as Chambers and Docktor stated, that the Stanford Sleepiness Scale (SSS) is fast and easy to administer. However, it is often difficult for subjects to select only one option within the SSS because the adjectives within each statement are not mutually exclusive from adjectives in the other seven options. When subjects select more than one option, their results must be eliminated from data analysis. Furthermore, with a categorical 7-point scale, variance is limited. The psychometric properties of the SSS should be examined and weighed against its ease of administration. If readers refer to Tables 4 and 5 (Lee et al., 1991), there are interesting data about the SSS: healthy subjects were more sleepy in the evening (mean = 4.0, SD = 1.59 on the SSS) than sleep disorder patients (mean = 3.0, SD = 1.64). In the morning, the mean SSS scores decreased to 2.4 (SD = 1.36) for healthy subjects and 2.7 (SD = 1.30) for patients. For research purposes, these small effect sizes require larger samples to detect statistically significant differences. Chambers and Docktor argue that distinguishing between fatigue and sleepiness is clinically important. That distinction is also important for researchers. Unfortunately, individuals may not be able to distinguish between sleepiness and fatigue with the precision that researchers and clinicians would like. Estimating any subjective state has its limitations. It is only through dialogue such as this that measurement issues become refined. Perhaps, with new samples and replication of studies, subscales specifically delin-
eating sleepiness from fatigue will emerge from the VAS-F. References Carmines, E.G., and Zeller, R.A. Reliability and Validity Assessment. Sage University Paper series on Quantitative Applications in the Social Sciences, series no. 07-017. Beverly Hills and London: Sage Publications, Inc., 1978. Krupp, L.B.; LaRocca, N.G.; Muir-Mash, J.; and Steinberg, A.D. The Fatigue Severity Scale. Archives of Neurology, 46: 112I- 1123, 1989. Lee, K.A.; Hicks, G.; and Nino-Murcia, G. Validity and reliability of a scale to assess fatigue. Psychiatry Research, 36:291-298, 1991. Rosekind, M.R.; Connell, L.J.; Dinges, D.F.; Rountree, M.S.; and Graeber, R.C. Preplanned cockpit rest: EEG sleep and effects on physiological alertness. Sleep Research, 20: 129, 199 1. Stepanski, E.; Zorick, F.; Roehrs, T.; Young, D.; and Roth, T. Daytime alertness in patients with chronic insomnia compared with asymptomatic control subjects. Sleep, 11:54-60, 1988. Kathryn A. Lee, R.N., Ph.D. Department of Family Health Care Nursing School of Nursing University of California, San Francisco San Francisco, CA, USA December IS. 1992