In contrast to previous studies which assessed DIF associated with children diagnosed with chronic conditions [27
], this study focuses on DIF associated with CSHCN. Specifically, we applied a MG-MIMIC method to identify uniform and non-uniform DIF. We found that CSHCN were associated with impaired HRQOL in raw item scores compared to children without needs. Seven items (30% of the total items in the PedsQL) were flagged with DIF; among the DIF items, five were non-uniform and two were uniform. For non-uniform DIF, factor loadings of the items were higher or lower between both groups, suggesting the DIF items possessed different abilities to distinguish under and above a certain level of underlying HRQOL between both groups. For uniform DIF, some threshold parameters of the items were smaller for CSHCN than children without needs, suggesting given a certain level of underlying HRQOL, parents of CSHCN were likely to report less problems in performing daily functioning than children without needs. The DIF finding, especially the non-uniform type, leads to the effect of DIF cancelled-out at the test level, as indicated by the expected test score function and test information function where the discrepancy between both groups was not overwhelmingly significant.
The impact of DIF on the score calculation of individual children and on the comparisons of HRQOL between both groups is also minimal. DIF calibration is associated with small amount of CSHCN and children without needs who changed domain scores by two points. After calibrating DIF items, the disparity in HRQOL scores on the affected domains was increased slightly between both groups.
Comparing this study to a previous study which only assessed DIF in social functioning of the PedsQL suggests that two DIF items were commonly flagged in both studies and two additional DIF item was identified in the previous study [28
]. Fifty percent of DIF items in the previous study were uniform and the remaining 50% were non-uniform; however, all DIF items in this study were non-uniform. This difference may be in part related to the use of different ways to classify children’s health status and different methods to identify DIF by both studies. This study classified children by a non-categorical approach based on the CSHCN Screener, focused on parent-proxy report of child’s HRQOL, and identified DIF by a MG-MIMIC method, whereas a previous study used a categorical/diagnosis-based approach, child self-report of HRQL, and a IRT-LR method.
The DIF findings can be interpreted from a cognitive and psychosocial point of view [45
]. Compared to children without needs, parents of CSHCN may change their internal standard (e.g., altering their expectations) when assessing their children’s daily functioning, such as assuming their children are not responsible for fulfilling all physical activities and social roles [46
]. After the illness journey and treatment, parents of CSHCN may be resilient to the uncertainty associated with illness and hold a positive outlook related to their ability to function with the illness, including being more positive, having a deeper appreciation for life, letting go of worry, and living for today [49
]. One qualitative study found that children with asthma tend to use different strategies to normalize their daily life, such as acknowledging the existence of asthma, minimizing the impact of asthma on health, emphasizing their ability to manage asthma and making adaptations in daily life [50
]. This resiliency finding emphasizes that psychosocial adjustment and adaptive coping processes may enable children with chronic conditions to maintain or improve their personal growth and HRQOL, despite a progressive, disabling disease [51
From a measurement perspective, DIF findings indicate a violation of the unidimensional assumption of the IRT [52
]. In our study, the unsatisfied fit of unidimensionality is salient in social and school functioning which is corresponding to more prominent DIF findings. This suggests the intercorrelated multidimensional phenomenon may exist in pediatric HRQOL measurement because, for the growth and developmental reasons, a child’s performance on social and school functioning will rely on his/her physical and emotional functioning. Our analyses suggest that the correlations among the four domains of PedsQL were between 0.46 and 0.60. When the data is multidimensional and a unidimensional model is applied, it is likely that item parameters will be distorted due to local dependencies caused by secondary factor and over or under representation of specific content domains. In this regard, the use of bi-factor model is helpful, especially if we conceptualize HRQOL as a dominant general factor and specific domains as group factors. A general factor represents a common trait which explains intercorrelations among items due to shared item contents. Group factors attempt to capture the item covariation that is independent of the covariation due to the general factor. This study echoes a previous study suggesting that if dimensions are moderately to highly correlated (≥
0.4), the bi-factor representation will be a useful alternative [37
A purification process for estimating the underlying HRQOL scores and the balance of covariates between groups are critical, but less discussed issues in DIF research. Purification is an iterative process using a two-step approach to estimate the underlying HRQOL scores (i.e., assessing DIF and then recalculating the underlying scores by accounting for DIF). Several studies found that identifying DIF in the first stage may change the DIF status in the second stage, and ignoring a purification process will lead to over- or underestimating the number of DIF items [21
]. Conducting a purification process is important because DIF tests emphasize the control for the underlying HRQOL in the modeling. Unfortunately, many previous DIF studies used DIF-free items in the recalculation of the underlying scores [53
]. The underlying HRQOL scores calculated by MIMIC method are purified given the fact that the parameters detecting and controlling for DIF are estimated iteratively during model building.
Covariate balancing may confound DIF detection and the subsequent comparisons of HRQOL between the groups. For example, if age of parents in the CSHCN group is younger than parents in children without needs, and the age is assumed to influence the conception of children’s HRQOL. Without balancing the age of parents in both groups, the DIF findings related to CSHCN may be confounded by the age of parents. This issue, however, has not been explicitly addressed in DIF algorithms, and the procedures for balancing covariates must be conducted in DIF tests. Although the use of a single group-MIMIC method can incorporate covariates in DIF tests, this method only detects uniform DIF [23
]. Our study is among very few studies [25
] demonstrate the usefulness of MG-MIMIC methods to investigate uniform and non-uniform DIF. Also, the use of a propensity score approach as demonstrated in this study to select matched subjects from both groups is a feasible method for DIF tests.
DIF investigation provides important insights for instrument refinement and psychological research. If we believe DIF represents biased items, instrument developers can use DIF information to guide item modification. By contrast, if we believe DIF findings reflect the results of psychosocial adjustment or true change of item meaning, item calibration between different groups of interest would be the best strategy. By item calibration, item parameters can be separately estimated for the subgroups, and these different parameter estimates can subsequently be used to estimate the underlying HRQOL. Nevertheless, further research is encouraged to use cognitive interviewing techniques to investigate the psychological mechanisms behind the DIF findings, especially how the DIF items are interpreted by CSHCN and children without needs themselves, and by parents of both groups of children [55
Some potential limitations merit attention. First, this study is restricted to children from low income families and enrolled in Florida KidCare, which will threaten the external validity and limit the generalizability of the findings to general children populations. This low income population, however, is important to assess because they are at a greater risk of chronic and life-limiting conditions due to their poor socioeconomic circumstances [56
]. Second, this study used the parent’s ratings of their child’s HRQOL, which may differ from the child’s or adolescent’s own ratings [38
]. Synthesized evidence suggests the agreement about the child’s HRQOL rated by the parent and child was greater on the observable domains (e.g. physical functioning) than on the unobservable domains (e.g. emotional, social, and school functioning) [58
]. Further studies based on ratings of children are needed to determine whether the same DIF can be replicated compared to parents’ ratings. Third, this study did not collect and explicitly control for parents’ psychological variables such as depression, which may confound the DIF detection if these variables were not balanced between both groups. This will threaten internal validity of the DIF findings. It is possible that parents’ ratings of pediatric HRQOL may have been influenced by their own mental health [59
In summary, although 30% of items in the PedsQL were flagged with DIF related to CSHCN and children without needs, the impact of DIF was negligible. Nevertheless, DIF assessment is useful for refining HRQOL instruments and investigating psychological adjustment associated with CSHCN. For comparing HRQOL, researchers should conduct DIF tests, calibrate DIF items, and then compare group differences.