1. Introduction
Parents whose children have a mental illness report compromised health-related quality of life (HRQL) and more caregiver strain or psychological stress [
1,
2]. Because epidemiological studies show that 20% of children have a mental illness [
3,
4], there is a large proportion of parents with, or at risk for, poor HRQL. Meta-analytic evidence suggests that parents of children with mental illness have significantly worse HRQL compared to parents of healthy children and population norms [
1]. Notably, this effect was moderated by child age—parents of older children with mental illness had worse HRQL compared to parents of younger children with mental illness. Understanding the contextual effects that contribute to compromised HRQL is important, because parents of children with mental illness take leading roles in caregiving, seeking and obtaining professional health services, and supporting the overall functioning of the family [
5,
6]. There are negative consequences for children with mental illness and families when the HRQL of their parents is compromised [
7,
8]. Addressing key questions of whether, why, and how parents are affected by caring for a child with mental illness can help direct resources and treatment programming using family-centered care frameworks within child and adolescent psychiatry.
However, given the unique, and often stressful, experiences of parenting children with mental illness, researchers and health professionals must ensure that the HRQL of these vulnerable parents is measured with sufficient validity and reliability. Such methodological work is needed so that meaningful comparisons can be made between this population of parents and parents of healthy children or parents of children with other chronic illnesses, as well as population norms to determine the extent to which child mental illness impacts parent HRQL.
The Short Form-36 (SF-36) is one of the most commonly used measures of adult HRQL [
9] and has a strong track record of use among parents of children with mental illness [
1]. However, no studies have assessed its psychometric properties in this population. Its pervasiveness in health research stems from the comprehensiveness of its development to conceptualize health as a multidimensional construct and to measure the range of health states, including well-being and individual perceptions of health. The SF-36 includes 36 items that measure eight health-related concepts or subscales: Physical Functioning (ability to perform physical activities), Role-physical (problems with activities as a result of physical health), Bodily Pain (level of pain/limitations), General Health (perception of overall health), Vitality (feelings of energy and fatigue), Social Functioning (ability to perform social activities), Mental Health (feelings related to internalizing problems), and Role-emotional (problems with activities as a result of mental/emotional health). These subscales define physical and mental summary component scores that accounted for approximately 80% of the variance in the SF-36 in the samples used in its initial development [
10,
11,
12]. Further, these component scores have proved useful in interpreting HRQL [
10,
11,
12]. Substantial evidence supports this two-factor structure of the SF-36, whereby the eight subscales (manifest variables) load onto the physical and mental summary components (latent variables), as well as indicators of its construct validity and reliability (i.e., internal consistency), in the general population [
13,
14], and in specific subpopulations of adults including pregnant women [
15], those with chronic physical illnesses [
16] and their caregivers [
17], and the elderly [
18].
This study assessed the psychometric properties of the SF-36 in a sample of parents whose children have mental illness. Specifically, our objectives were to replicate the two-factor structure of the SF-36 (eight subscales loading onto the two summary components); examine internal consistency of the SF-36 and inter-subscale correlations; and, investigate construct validity of the SF-36 with self-reported measures of psychopathology (depression, anxiety, parental stress) and child diagnoses of mental illness. It was anticipated that a two-factor (physical and mental) correlated structure would be confirmed and that based on our sample size and number of items in each subscale, internal consistency reliabilities of the SF-36 would be “good” (α ≥ 0.70) [
19]. We did not expect to find age or sex differences in SF-36 scores in our sample of parents. Furthermore, it was expected that correlations with measures of psychopathology would be statistically significant and at least small in magnitude (r ≥ 0.20) and larger for the mental versus the physical component score. We hypothesized that significantly lower SF-36 scores would be found in parents with clinically relevant symptoms of depression or anxiety and that parent SF-36 scores would be inversely associated with the number of mental illness diagnoses in their children in a dose–response manner.
3. Results
3.1. Sample Characteristics
Parents were, on average, 45.3 (SD 6.7) years of age and the majority were female (n = 84; 85%). Nearly two-thirds (n = 60; 61%) had a partner (i.e., married or common-law) and had completed postsecondary education (n = 65; 66%). Half of parents (n = 49) reported an annual household income of at least CAD 75,000; the median household income according to the 2016 Canadian Census. Immigrants were underrepresented in this sample of parents (n = 13; 13%).
Children had a mean age of 13.9 (3.1; range 8–17) years and 71% (n = 70) were female. Parent physical and mental component scores were not associated with child age or sex. The most common illnesses affecting children were major depression and generalized anxiety (n = 72; 73% each). The remaining diagnoses were: phobia (71%), oppositional defiant (47%), attention-deficit hyperactivity (33%), separation anxiety (32%), and conduct disorder (25%). As typical in child and adolescent psychiatry, comorbidity was prevalent: 15%, 26% and 46% of children had two, three, or four or more mental illnesses. Overall, 38 (38%) of children were recruited from the inpatient setting. While physical component scores were similar between parents of children receiving inpatient vs. outpatient services (40.64 vs. 43.81; p = 0.122), parents of inpatient children had significantly lower mental component scores to those of parents of outpatient children (35.93 vs. 40.21; p = 0.035).
Descriptive statistics for SF-36 subscale and component scores are shown in
Table 1. Mean parent scores were lowest for the Role-emotional subscale [37.42 (13.25)] and highest for the Physical Functioning subscale [48.16 (11.36)].
3.2. Factor Structure
We tested three factor structures of the SF-36 and the fit indices for these models are shown in
Table 2. Prior to fitting the factor models, baseline models of the physical and mental components were specified to ensure adequate fit. Both components showed adequate fit to the data, though the physical component model had lower (i.e., better) SRMR (0.06 vs. 0.08) and RMSEA (0.26 vs. 0.33) indices. The one-factor model in which all subscales were loaded onto a single HRQL factor fit the data poorly, with none of the fit indices achieving an adequate fit threshold. The two-factor model which did not specify a correlation between the physical and mental component factors also had inadequate fit to the data; again, none of the fit indices achieved an adequate fit threshold. The two-factor model which specified a correlation between the physical and mental component factors showed adequate fit to the data: CFI = 0.94 and SRMR = 0.07. It also demonstrated significantly better fit according to the χ
2 difference test against the one-factor (χ
2 = 79.1;
p ≤ 0.001) and two-factor uncorrelated models (χ
2 = 167.1;
p ≤ 0.001).
3.3. Internal Consistency
Internal consistency reliabilities of the SF-36 subscales were generally robust (α > 0.80), with the exception of General Health (α = 0.65), Vitality (α = 0.59), and Mental Health (α = 0.64), which were lower that established guidelines indicating good reliability. Internal consistencies of the physical and mental components were very strong (α = 0.91 and α = 0.85, respectively). Relatedly, correlations among subscales of the SF-36 were also robust (
Table 3); correlations that were statistically significant (
p < 0.01) ranged from a low of r = 0.31 between General Health and Role-emotional to a high of r = 0.65 between Physical Functioning and Bodily Pain. Non-significant correlations were found between Physical Functioning and Vitality (r = 0.06), Role-emotional (r = 0.23), and Mental Health (r = 0.20). The correlation between Bodily Pain and Vitality was also not significant (r = 0.23). The correlation between the physical and mental components was very strong (r = 0.75).
3.4. Construct Validity
The SF-36 demonstrated no significant correlation with parent age for both the physical component (r = −0.08; p = 0.464) and mental component (r = −0.03; p = 0.774) scores. Further, differences between mothers and fathers were not statistically significant—physical: 45.39 (8.62) vs. 42.09 (10.09); t = 1.19; p = 0.237 and mental: 40.08 (9.34) vs. 38.30 (9.98); t = 0.64; p = 0.523.
Convergent validity of the SF-36 was tested by estimating the strength of the correlation between measures of parent psychopathology—symptoms of depression, anxiety, and parental stress—with both the physical and mental component scores (
Table 4). Correlations were all statistically significant (
p < 0.001) and generally medium-sized in magnitude. While correlations were larger with the mental component score, the differences were not statistically significant compared to the physical component score. Using established thresholds for the CESD and STAI, we dichotomized our sample of parents as having or not having clinically relevant symptoms of depression or anxiety to determine if SF-36 scores would be different between these groups of parents. As shown in
Table 5, parents with elevated CESD or STAI scores had significantly lower physical and mental component scores.
We then tested the extent to which SF-36 component scores in parents were associated with the number of mental illness diagnoses in children (
Table 6). Both physical and mental component scores decreased significantly with increases in the number of child diagnoses (F = 7.68;
p < 0.001 and F = 6.10;
p = 0.001, respectively). Post hoc comparisons showed that parents of children with one mental illness had significantly higher SF-36 physical component scores compared to parents of children with three or four mental illnesses (
p = 0.015 and
p < 0.001, respectively). Parents of children with two diagnoses had significantly lower physical components scores compared to parents of children with four diagnoses (
p = 0.026). With regard to the mental component, parents of children with one mental illness had significantly lower scores compared to parents of children with three or four mental illnesses (
p = 0.038 and
p = 0.001, respectively).
4. Discussion
This study aimed to replicate the factor structure of the SF-36, estimate internal consistency and inter-subscale correlations, and describe its construct validity with measures of parent psychopathology and child mental illness. Evidence from this study supported our hypothesis that the SF-36 has a correlated two-factor structure and corroborates a previous large-scale study investigating the factor structure of the SF-36 [
14]. This correlation is intuitive as there is ample evidence supporting the reciprocal link between physical and mental health across the life course [
43,
44,
45,
46]. Previous studies have tested the fit of additional models that include a four-factor [
47], higher-order factor [
15], and bifactor [
48] solutions. We did not investigate such models for two reasons. First, our sample size lacked the statistical power to reliably estimate such complex models. Second, the two component scores of the SF-36 are most commonly used in research and clinical practice [
49] and that use of an overall HRQL score based on the SF-36 still warrants further validation in large-scale population studies [
50].
Reliability was very strong for both the physical and mental components and most of the eight subscales. Internal consistencies were considered “moderate” for the General Health and Mental Health given the number of items for each subscale and our sample size [
19]. Vitality was considered “unsatisfactory” as a large proportion of its variance, approximately 40%, was due to measurement error. Thus, in the context of parents whose children have mental illness, this subscale has limited predictive utility and should be used with caution. Our hypothesis regarding the correlations among SF-36 subscale and component scores was partially supported. All correlations were in the expected direction and magnitude. This finding provided further evidence supporting the two-factor structure of the SF-36 in this population of parents.
Discriminant validity of the SF-36 was demonstrated with similar physical and mental component scores across age and sex. Despite evidence of age and sex differences in the SF-36 among population norms [
25], our null findings were expected. First, variability in the age of our sample of parents was relatively small. Second, reported age differences in population studies are small and do not necessarily reflect clinically relevant differences in HRQL [
25,
51]. Third, given the context in which we studied our sample—eligible parents were those who were the primary caregiver for their children—comparisons may better reflect gender role, rather than sex. Previous reports have shown that among parents of children with a chronic illness, strain and psychological distress is similar among primary caregiving fathers and primary caregiving mothers [
52].
Convergent validity of the SF-36 was also demonstrated. Findings showed that the mental component of the SF-36 taps into symptoms of internalizing disorders better than generic caregiver strain. While stronger correlations were found for the mental versus physical component score, these differences were not significant and reaffirm the physical-mental health link. Similar findings were seen when comparing parents with versus without clinically relevant symptoms of psychopathology and which has been reported previously [
53].
Finally, worse HRQL in parents of children with multiple mental illnesses was supported. This inverse dose–response association was linear in nature and likely reflects the challenges experienced by parents caring for a child with multiple illnesses; a common occurrence in child and adolescent psychiatry [
4]. The number of comorbidities can also be a proxy for illness severity. Inverse associations between child illness severity and parent HRQL have been reported in samples of children with physical [
54] and mental illnesses [
55,
56].
4.1. Study Implications
The findings contribute to extending the validity and reliability of the SF-36 and should provide confidence using this measure in parents of children with mental illness. Replication of its factor structure suggests that the HRQL construct is interpreted in this sample of parents in the same manner in which the SF-36 was initially developed. Thus, it can be used to make comparisons across samples and with population norms. The design of the current study did not allow for the examination of test–retest reliability and predictive validity of the SF-36. Given the uniqueness of this parent population, these important psychometric properties should be evaluated in the future studies. Additional work is needed to demonstrate that the SF-36 has clinical utility as a patient-reported outcome assessing change in HRQL. Quantifying its responsiveness to change and calibrating minimally important differences is necessary prior to implementing the SF-36 as a potential outcome in the evaluation of family-centered approaches to care within child and adolescent psychiatry. Further, given the strong correlation between the mental component score and the CESD (and to a lesser extent, the STAI), it appears as though the same underlying construct, internalizing symptoms, is being measured [
57]. In conducting future studies, researchers may consider including the SF-36 to measure both HRQL and parent psychopathology in order to reduce respondent burden.
4.2. Limitations
There are notable limitations to this study. First, the relatively small sample size prevented the examination of more complex factor structures that could potentially result in a better fitting model (e.g., bifactor) and underrepresentation of immigrants [
58] may limit the generalizability of the findings to this population of parents of children with mental illness. Relatedly, recruitment from a single site may not adequately reflect the broader experience of this population. Second, shared method variance may have resulted in overestimated correlations and associations. Third, the absence of a control group (i.e., parents of children without mental illness) prevented the opportunity to formally test for measurement invariance of the SF-36, further contributing to its validation in this population.