Distinguishing between frequency and intensity of health-related symptoms from diary assessments

https://doi.org/10.1016/j.jpsychores.2014.07.006Get rights and content

Highlights

  • We ask if symptom frequency and intensity should be viewed as distinctive outcomes.

  • A statistical “two-part” model is applied to multiple diary ratings.

  • Symptom frequency and intensity can be reliably distinguished using this strategy.

  • Differential associations with demographic factors suggest utility of the approach.

Abstract

Objective

This study investigated the utility of distinguishing between the frequency and intensity of self-reported symptoms using diary-based assessments in a representative sample of U.S. residents.

Methods

Data from the 2010 American Time Use Survey were analyzed, in which 12,000 respondents provided a diary about the prior day and rated their pain, tiredness, stress, and sadness for three of the day's episodes. A “two-part” latent variable modeling strategy was applied to estimate the frequency (propensity of its presence) and intensity (mean level when present) of each symptom from the diary ratings. Regression analyses comparing differences in symptom frequency and intensity across demographic factors (gender, age, income, education) were conducted to evaluate the utility of the distinction.

Results

Frequency and intensity measures were reliably estimated from 3 daily episodes, were moderately intercorrelated for each symptom domain (rs .39 to .60), and were differentially associated with demographic factors. Gender differences were evident only in symptom intensity, not frequency, with women reporting more intense symptoms. Comparisons by age showed pronounced declines in the frequency of tiredness and stress in older age, with no age-differences in the intensity of these symptoms. Higher socioeconomic status was associated with a lower intensity of pain, tiredness, stress, and sadness, but a higher frequency of tiredness and stress.

Conclusion

A useful distinction between symptom frequency and intensity may be made from diary-based assessments. It reveals demographic differences that are otherwise obscured and enables a more detailed characterization of health-related experiences in people's daily life.

Introduction

Interest in self-reported somatic and affective symptoms is high in research, clinical, and health policy settings. Knowledge about health-related symptoms is important for evaluating health care and treatment, for understanding health disparities, and for tracking population trends in health and wellbeing over time [1], [2]. To date, two characteristics of symptom experience – their frequency and their intensity – have often been overlooked or simply combined into single measures, in part, because there has been limited empirical study of the distinction [3]. The question addressed here is whether symptom frequency and intensity should be viewed and can be measured as distinctive health outcomes.

There are compelling conceptual arguments for separating the frequency and intensity of symptom experiences. A person could have symptoms of pain, fatigue, or emotional distress at mild levels yet very often, whereas another person could have symptoms at high levels but only occasionally, as in the case of symptom flares. The overall symptom severity (i.e., its average magnitude across time) could be very similar for both people, despite pronounced differences in the composition of symptom frequency and intensity. Discriminating these patterns could have implications for practice and research, perhaps suggesting different mechanisms and indicating different treatment strategies [3], [4].

Despite its theoretical appeal, the frequency–intensity distinction has received little empirical justification in past research on self-reported somatic symptoms. Chang et al. [3] compared retrospective self-report ratings of fatigue using a frequency (none of the time–all of the time) or intensity (not at all–very much) response format and found that the two produced largely corresponding scale scores (correlation of .86). Similarly, scales that ask participants to rate either the frequency (not at all–almost always) or intensity (not at all–extremely) of posttraumatic stress disorder symptoms have been found to yield highly overlapping information (correlation of .93) [5]. Based on these findings, it has been argued that the concepts are virtually redundant and that there is little use in querying frequency and intensity of somatic symptoms separately [3], [4], [5].

Importantly, however, these studies examined retrospective questionnaires, where respondents were asked to summarize their symptoms over several days (e.g., the past 7 days)[3]. Recall ratings can be impacted by memory biases [6], and contextual factors can influence how people use and interpret frequency and intensity response scales in retrospective self-reports [7]. Symptom diaries, such as ecological momentary assessment (EMA) and the Day Reconstruction method (DRM), mitigate or eliminate the effects of recall bias [6], [8]. In addition, by collecting experience ratings across multiple moments or episodes, outcome measures that summarize these experiences are created by the researcher and not implicitly by the respondents. In other words, measures of symptom frequency and intensity can be directly computed from the diary data instead of relying on the participant's ability to meaningfully map their experiences onto a response scale that queries either frequency or intensity [9].

The purpose of this study was to investigate whether symptom diaries allow for a reliable and useful distinction between the frequency and intensity of health-related outcomes. Data from a nationally representative sample of over 12,000 individuals collected by the 2010 American Time Use Survey (ATUS, http://www.bls.gov/tus/) were utilized. Similar to the DRM, respondents were interviewed about the prior day, provided a “chunking” of the day into distinct episodes, and rated their pain, tiredness, sadness, and stress for 3 selected episodes. We conceptualized the frequency of a symptom as the proportion of episodes in which it was endorsed as present, and its intensity as the average level of the symptom when it was present, consistent with prior related literature examining basic components of affect [10], [11], [12].

To evaluate the utility of the distinction, we examined the extent to which symptom frequency and intensity were differentially associated with demographic characteristics, notably, gender, age, income, and educational attainment. Insight into the prevalence of emotional and somatic symptoms across demographic groups is important for understanding of who is more likely to seek healthcare and to facilitate more cost-effective utilization of healthcare resources. However, it is important that the derived prevalence rates be as precise and informative as possible. Thus, the question we addressed here was whether separating frequency and intensity symptom components reveals demographic differences that are otherwise obscured.

Section snippets

Participants and procedure

Data collected as part of the US Bureau of Labor Statistics' 2010 ATUS project with addition of the NIA-supported Wellbeing Module (WBM) were used for this study. The main purpose of ATUS is to develop nationally representative estimates of how people spend their time based on a subset of households who recently completed the Current Population Survey (CPS). Respondents are interviewed over the telephone to provide a detailed time diary of the previous day. In a series of questions, the

Descriptive results

Health-related symptom data were available from 12,829 ATUS Wellbeing Module respondents. Participants provided symptom ratings for 38,059 diary episodes or an average of 2.97 episodes per person, slightly less than the targeted 3 episodes per person (1.1% responses were missing). The mean episode length was 67 min (SD = 96, range 5 to 1107 min). Fig. 2 shows the response distribution for pain, tired, sad, and stress. As can be seen, the distributions were heavily skewed with a large proportion of

Discussion

The results of this study support the distinction between the frequency and intensity of health-related symptoms in the context of a multi-episode, diary-based assessment method. Both symptom components could be captured with reasonably good reliability of > .70 based on no more than 3 diary episodes collected in the ATUS wellbeing module. Notably, the reliabilities were similar to those obtained when applying the traditional strategy of averaging the ratings, suggesting that decomposing symptom

Conflict of interest

A.A.S. is a Senior Scientist with the Gallup Organization and a Senior Consultant with ERT, Inc.

Acknowledgments

This research was supported by a grant from the National Institute on Aging (P30 AG024928; Stone, PI).

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