QUANTITATIVE METHODS SPECIAL SECTION Energy, fatigue, or both? A bifactor modeling approach to the conceptualization and measurement of vitality Nina Deng • Rick Guyer • John E. Ware Jr. Accepted: 24 October 2014 / Published online: 2 November 2014 Ó Springer International Publishing Switzerland 2014 Abstract Purpose Vitality is an important domain reflecting both the physical and emotional components of health-related quality of life. Because of its complexity, it has been defined and measured both broadly and narrowly. We explored the dimensionality of a very comprehensive item bank hypoth- esized to measure vitality and its related concepts. Methods Secondary analyses were conducted using the responses of 1,343 adults representative of the US general population to Internet-based surveys including 42 items compiled from multiple scales (e.g., SF-36 Vitality, PRO- MIS-Fatigue), covering a broad range of vitality-related content areas (energy, fatigue, and their interference with physical, mental, social activities, and quality of life). Exploratory and confirmatory factor models were evalu- ated independently using split-half samples. Bifactor model was used to assess the essential unidimensionality of the items, in comparison with traditional unidimensional, multidimensional, and hierarchical models. Method effects of a common scale or phrase were modeled via correlating errors. Results The exploratory factor analysis identified one dominant factor. The confirmatory factor analysis identi- fied a best-fitting (CFI = 0.964, RMSEA = 0.084) bifac- tor model with one general (vitality) and two group (energy and fatigue) factors, explaining 69, 3, and 4 % of total variance. Correlating errors accounting for the method effects were important in identifying the substantive dimensionality of the items. Conclusions The bifactor model proved to be useful for evaluating the dimensionality of a complex construct. Results supported conceptualizing and measuring vitality as a unidimensional energy-fatigue construct. We encour- age future studies comparing practical implications of measures based on the broader and narrower conceptual- izations of vitality. Keywords Bifactor model Á Dimensionality Á Factor analyses Á Vitality Á Fatigue Á Quality of life Introduction Vitality is defined as energy, stamina, and physical or mental vigor [1]. It is an important concept shown to reflect both the physical and emotional components of health and is frequently measured in health-related quality-of-life (HRQOL) surveys [2–7]. The content of vitality includes conceptually related symptoms such as fatigue and lack of energy. These symptoms are not disease specific and are commonly reported among patients with cancer, diabetes, heart diseases, depression, and other chronic conditions [8, 9]. These symptoms are also reported by relatively healthy people; for example, 38 % of US workers reported being fatigued in 2007 [10]. Further, vitality measures proved to be responsive to the (un)desirable side effects of drugs and are a common health outcome in clinical N. Deng (&) Á J. E. Ware Jr. Quantitative Health Sciences Department, University of Massachusetts Medical School, Worcester, MA 01655, USA e-mail: deng.nina@measuredprogress.org J. E. Ware Jr. e-mail: John.Ware@jwrginc.com N. Deng Measured Progress, Inc., 100 Education Way, Dover, NH 03821, USA R. Guyer Á J. E. Ware Jr. John Ware Research Group, Incorporated, One Innovation Drive, Suite 400, Worcester, MA 01605, USA e-mail: Rick.Guyer@jwrginc.com 123 Qual Life Res (2015) 24:81–93 DOI 10.1007/s11136-014-0839-9