Population
Maharaj Nakorn Chiang Mai Hospital, participated in the study. All gave written informed consent before completing the questionnaires. Sociodemographic data and records related to caregiving were obtained in addition to specific measurement.
Measurement
ZBI
The ZBI is a caregiver-reported questionnaire measuring the burden the respondent feels in providing caregiving to the patient. Currently, it has two forms, long (22 items) and short (12 items), with a Likert scoring scale between 0 (never) and 4 (nearly always). The ZBI offers the interpretation of score as follows; 0 to 20, little or no burden, 21 to 40, mild to moderate burden, 41 to 60, moderate to severe burden and 61 to 80, severe burden[4, 15].
The Thai version of the ZBI was allowed to be used for the present study by Professor Zarit and Mapi Research Trust[16]. The study sample showed a Cronbach’s alpha of .921 for the full-length version, and .865 for the short version.
Perceived stress scale (PSS)
The PSS is a self-reporting, 10-item questionnaire measuring the extent to which individuals perceived stress [17]. The 4-response Likert scale, ranges from 0 (not at all) to 4 (the most); the total scores ranges from 0 to 40 and the higher the score, the higher the level of feeling stress.
The Thai version showed good psychometric properties [18]. The study sample showed a Cronbach’s alpha of .850.
Patient Health Questionnaire (PHQ-9)
The PHQ-9 is a self-reporting, 9-item questionnaire measuring the extent to which an individual feels bothered due to depressive symptoms over the past two weeks [19]. The 4-response Likert scale ranges from 0 (not at all) to 3 (nearly every day); the total scores ranges from 0 to 27 and the higher the score, the higher the level of feeling depression. The Thai version showed good psychometric properties [20]. The study sample showed a Cronbach’s alpha of .849.
EQ-5D
The EQ5D is a self-reporting questionnaire measuring health-related quality of life [21]. It comprises a 5-item questionnaire assessing 5 domains of health state: mobility, self-care, usual activities, pain and anxiety/depression, with a 5-response type ranging from 1 (no problem) to 5 (severe problem). All 5 aspects were calculated to an index score, with the maximum of 1.000, the higher the score, the better the quality of life. The utility values for EQ-5D health states were estimated from the Thai general population [22]. The study sample showed a Cronbach’s alpha was .723.
Statistical Analysis
Sociodemographic data was analyzed using descriptive statistics. Pearson or Spearman rank was used for correlational analysis. The same items are present in both tests, leading to an overestimate of the "true" correlation, so a corrected correlation was made between the full length and short forms ZBI[23].
Based on measurement theory, a scale should demonstrate that all items contribute to the same construct, and has monotonically increasing steps. All these properties can be illustrated by the Rasch model. The following approach was conducted for analysis.
Rasch analysis
The Rasch model belongs to the item-response latent trait models, probabilistic logistic model that predicts that the response to a particular item is influenced by the quality of both person and item. To investigate the ZBI data, the partial credit Rasch model was used [24], with the following criteria. First, unidimensionality and local independence, which were evaluated by a) the first principal component of the residuals (PCA) (or first contrast) should have an eigen value less than 2, b) disattenuated correlation > 0.7, and c) item fit statistics (inlier-pattern-sensitive-INFIT, outlier-sensitive fit -OUTFIT mean-square) indicating the consistency of each item to the other items, should be .70 and 1.30 [25]. To evaluate local independency, an inter-item residual correlation should be less than 0.3 [26]. Second, response category functioning and ordered categories and thresholds are expected for measurement. [27, 28]. Third, for reliability, a reliability coefficient of 0.80 or higher and of 0.90 or higher are considered acceptable for person reliability and item reliability, respectively.
Convergent, discriminant and concurrent validity
We tested the ZBI against the EQ5D subscale, hypothesizing that ZBI should relate more to anxiety/depression than mobility. We expected to find a positive correlation between ZBI and PSS and PHQ-9 to demonstrate concurrent validity.
Confirmatory factor analysis (CFA)
To test how data were well modeled with the unidimensional construct, CFA was performed for both ZBI-22 and ZBI-12. The Weighted Least Square Mean and Variance corrected (WLSMV) method of estimation was used for the nonnormality and ordinal types of items. Assessment model fit used Chi-square (p > .05), comparative fit index (CFI) and Tucker Lewis Index (TLI), where values 0.95 or higher are preferable [29]. Root mean square error of approximation (RMSEA) was also performed. A RMSEA value <0.08 was indicative of an acceptable model fit [30]. Internal consistency was determined using Cronbach’s alpha.
For CFA, Mplus, Version 8.4 was used (Muthén and Muthén 2015). All the other analyses were performed using IBM SPSS, Version 22 (SPSS Inc., Chicago, IL, USA).