Summarizing health-related quality of life (HRQOL): development and testing of a one-factor model

This article is distributed under the terms of the Creative Commons Attribution 4.0 International License ( http://creativecommons.org/licenses/by/4.0/ ), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver ( http://creativecommons.org/publicdomain/zero/1.0/ ) applies to the data made available in this article, unless otherwise stated.

Instead of using four individual items to measure HRQOL, it is feasible to study overall HRQOL via factor analysis with one underlying construct. The resulting summary score of HRQOL may be used for health evaluation, subgroup comparison, trend monitoring, and risk factor identification.

Both exploratory factor analysis and goodness of fit tests supported the notion that one summary factor could capture overall HRQOL. Confirmatory factor analysis indicated acceptable goodness of fit of this model. The predicted factor score showed good validity with all of the four HRQOL items. In addition, use of the one-factor model showed stability, with no changes being detected from 2001 to 2013.

Exploratory and confirmatory factor analyses were performed using BRFSS 2013 data to determine potential numerical relationships among the four HRQOL items. We also examined the stability of our proposed one-factor model over time by using BRFSS 2001–2010 and BRFSS 2011–2013 data sets.

Health-related quality of life (HRQOL) is a multi-dimensional concept commonly used to examine the impact of health status on quality of life. HRQOL is often measured by four core questions that asked about general health status and number of unhealthy days in the Behavioral Risk Factor Surveillance System (BRFSS). Use of these measures individually, however, may not provide a cohesive picture of overall HRQOL. To address this concern, this study developed and tested a method for combining these four measures into a summary score.

Background

Health-related quality of life (HRQOL) is a useful indicator of overall health because it captures information on the physical and mental health status of individuals, and on the impact of health status on quality of life [1, 2]. HRQOL is usually assessed via multiple indicators of self-perceived health status and physical and emotional functioning. Together, these measures provide a comprehensive assessment of the burden of preventable diseases, injuries, and disabilities [3].

To assess and measure HRQOL at the state and national levels, the Centers for Disease Control and Prevention (CDC) developed a set of four “core” questions (CDC HRQOL-4): (1) Would you say that in general your health is excellent, very good, good, fair, or poor? (2) Now thinking about your physical health, which includes physical illness and injury, for how many days during the past 30 days was your physical health not good? (3) Now thinking about your mental health, which includes stress, depression, and problems with emotions, for how many days during the past 30 days was your mental health not good? (4) During the past 30 days, for about how many days did poor physical or mental health keep you from doing your usual activities, such as self-care, work, or recreation? [3–5].

These four items, which have demonstrated good retest reliability, validity, and responsiveness [6–8], have been included in the Behavioral Risk Factor Surveillance System (BRFSS) in all 50 states since 1993. In addition, the four items have also been included in other national surveys (e.g., National Health and Nutrition Examination Survey (NHANES), Medicare Health Outcome Survey) and in various chronic disease assessments [7, 9, 10]. CDC HRQOL-4 account for similar variance as the Patient-Reported Outcome Measurement Information System (PROMIS) items (e.g., SF-36) [11–13]. However, the CDC items appear more appropriate for assessing burden of disease for chronic conditions and are brief and easily interpretable [11].

In 1995, CDC added five additional questions related to quality of life to BRFSS, as part of an optional module. The new questions asked about days experiencing pain, feeling sad or depressed, feeling worried or anxious, not getting enough rest, or feeling healthy. However, the optional module was only used in a limited number of states and years.

To assess HRQOL comprehensively, public health professionals have sought a means to summarize these HRQOL measures. To combine the information on physically and mentally unhealthy days, some researchers have summed the two measures in CDC HRQOL-4 to create an Unhealthy Days Index, with the sum of the two items being truncated at 30 days [3, 14, 15]. This approach assumes an independent relationship between the two kinds of days.

Another approach is to view HRQOL as a latent (hidden) construct that can be quantified through factor analysis. Factor analysis is a method for detecting relationships among variables, which often reduces the number of variables. Previous studies found strong associations among the CDC HRQOL-4 questions, suggesting that these items may be suitable for factor analysis [4]. Toet and colleagues found good internal consistency of the four measures (the Cronbach’s alpha for the three unhealthy day measures was 0.77; a Cronbach’s alpha of 0.70 or more is usually considered acceptable [16]) [13]. Horner-Johnson and colleagues, on the other hand, found a relatively poor consistency between the mentally unhealthy day item and the three other items based on “the Cronbach’s alpha increase if item removed” test [17]. They compared two alpha values: one based on all items; the other based on remaining items after a test item was removed. This analysis relies on the premise that if the test item value increases, this may indicate poor consistency of the removed item. However, due to the lack of a clear cutoff value for the increase, it is a somewhat subjective choice to remove a single item measure, especially for situations in which the increase in the alpha values is minimal. Horner-Johnson and colleagues found only a very slight increase (e.g., 0.001 when using BRFSS 2002 data), which may not be enough to undermine the internal consistency of the mentally unhealthy day item with other HRQOL items [17]. Raykov and colleagues warned that the Cronbach’s alpha if item is removed test can be misleading for selecting construct components [18, 19].

Two studies have conducted HRQOL factor analysis using the CDC HRQOL-4 plus the five optional HRQOL module questions [7, 17]. Using data from BRFSS (2001 and 2002), both studies demonstrated that the nine HRQOL questions have good internal consistency and could be reduced to two latent factors that correspond to the physical and mental health aspects of HRQOL. However, data from the optional BRFSS module were only available for a few states and years, which limits the application of these models in tracking HRQOL over the years or assessing HRQOL at the national level.

This study proposes a method for creating a summary score of overall HRQOL based solely on CDC HRQOL-4. Public health professionals could treat such a consolidated score as a “new” variable that could be used to describe both community and population health, assess health disparities, monitor trends, and identify risk factors of overall HRQOL at the local and/or national levels. Using the 2013 BRFSS data set, the study assesses whether there is an underlying latent construct of HRQOL for the general population, and investigates the possibility of reducing CDC HRQOL-4 to one summary score. It also provides an example of how this type of summary score could be used in trend analysis using BRFSS 2001–2010 and 2011–2013 data sets.