This study has shown how a three-step model consisting of internal consistency, longitudinal confirmatory factor analysis, and mixed model analysis may be useful for the shortening of validated questionnaires while retaining most of the included variance and validity when having longitudinal data. Questionnaires that easily comprehend with few items are essential to ensure high completion rates among people with SUDs. Our findings showed that the full-scaled FSS-9 had excellent internal reliability when tested empirically by using internal consistency. However, a three-item FSS did not reduce substantially the internal consistency compared to the FSS-9 with or without VAFS. The results from the longitudinal confirmatory factor analysis of FSS-3 showed a well-fitted unidimensional model with equal factor loadings and equal residuals between baseline and second measurement. The FSS-9 and FSS-3 were almost perfectly correlated. The mixed model analysis showed that even though the level of fatigue varied, the scoring structure was substantially stable and equal between the measurements.
The reliability analyses showed high internal consistency of FSS-9 and FSS-3, and the internal consistency dropped minimally from 0.94 to 0.87 when reducing the number of items from ten (FSS-9 + VAFS) to three (FSS-3). A homogenous and substantial equal internal consistency was also found in studies evaluating FSS-9 in other chronic diseases such as stroke [15], hepatitis C virus infection [13], and multiple sclerosis [29]. A shortened FSS on seven items has also been validated on patients with systemic lupus erythematosus, HIV, or stroke to have minimal change on reliability compared to the FSS-9 [19, 21, 30]. Unlike other populations, people with SUDs may have a broad specter of mental and physical diseases that could interfere with the people’s experience of fatigue from measurement to another [31, 32]. The majority of people included in this study were marginalized, having more than one SUD, and more than 40% had injected drugs in the past 30 days. Consequently, people could be affected by centrally acting stimulants or sedatives taken immediately before the questionnaires were filled out, or some people could have substantial changes in their social conditions such as needing to move from housing to living on the street or changes in their income, which may impact the way they responded to fatigue. These psychosocial and medical conditions made the FSS-3 appropriate for ensuring credible results of items that were well-comprehended for people and considered whether the levels of functioning or self-perceived sense of fatigue were affected.
The confirmatory factor analyses demonstrated a unidimensional model at baseline and the second measurement for FSS-9 that was improved by adding residual covariance between items two and three. This pointed towards that the responses in these two items not adequately were captured by the one-factor model. The explanations could be that the Norwegian version had substantially similar wording of items two and three (Supplementary table 2), as well as the order of items in the questionnaire that affected people’s perception of them. Unidimensional factor models are frequently reported when validated FSS on other populations, including the general populations [16, 23, 30]. However, previous studies have noted that multidimensional models in confirmatory factor analysis could be missed due to small study populations [16, 30]. Our findings on a large population on people with SUDs confirmed that one single factor for analyzing FSS-3 matched the observations adequately. Nevertheless, residual covariance between items two and three was of importance for maintaining a well-fitted unidimensional model in FSS-9.
The linear mixed model analysis showed that four of the items in FSS-9 and the VAFS changed differently between measurements compared with other items. This gives further arguments for better validity of the shortened version, which is assumingly less sensitive for fluctuations than the remaining items in FSS-9.
Nine-item FSS may be shortened to FSS-3 without VAFS when evaluating fatigue among people with SUDs. However, FSS-3 may increase the risk of common method bias compared to FSS-9 if the responses to items in a shorter questionnaire are more likely to be accessible in short-term memory and recalled when responding to other items [33]. Previous studies have validated a shortened FSS-9 on seven items across people with multiple sclerosis, stroke, and Human immunodeficiency virus (HIV)/acquired immune deficiency syndrome (AIDS) showing high reliability, and however: present cross-sample differences in items three, five, six, and nine [19, 20]. Nevertheless, the items may have varying importance in different populations. Therefore, the construct validity of the FSS-3 may be limited to people with SUDs pending on validation studies that confirmed similar results on other populations.
Strengths and limitations
This study has several strengths. People who were included from the cohort are hard to reach for both research and health care. The majority of participants had an extensive history of injecting drug use with several drug addictions, and a substantial proportion had an ongoing injecting drug use. We plotted all collected information into the software directly for reducing the risk of data entry errors and ensuring nearly complete data for each measuring point. The data within each interview were also mostly complete with a few missing items.
The study has some limitations. People recruited to this study filled out the FSS and VAFS questionnaires in different mental and physical health conditions. Changes in substance use, medical and psychosocial challenges may impact on significant variations within and between people. Further, many participants were recruited from people with opioid dependence who received opioid agonist treatment while others, to a more substantial degree, utilized street drugs. In addition, there was some variation in time between the baseline and the second and third measurements. To adjust for this, those who fulfilled FSS and VAFS thrice, only one of the two last measurements that were closest to date one year after the baseline date, were included in the reliability analysis and confirmatory factor analyses. Further, common method bias could affect the shortening from nine to three items in FSS. A shorter questionnaire is more likely to be accessible in short-term memory and recalled when responding to other items [33].