In longitudinal studies, the fundamental assumption is that the measures are interpretable across time; however, when an RS occurs, this assumption is invalidated because an RS makes change difficult to assess. It is not uncommon for MS patients to report improved mental health status despite severe impairments in physical functioning
. When an RS is present, conventional statistical analyses might not detect true change in the measures
[26, 27]. It is critical for researchers and clinicians to have access to methods for detecting the presence of RS in their data. While several methods were previously used for this purpose, to our knowledge, this is the first study that assesses RS detection using the RF method. The RF method identified patterns of an RS in a global QoL change score. The reprioritization aspect of the RS was recognized through the qualitative differences of the importance of QoL specific domains that were retained by RF analysis.
Using the RF method, the RS was well identified in our worsened population. In this group, we observed that the mental composite score became more important during the twelve months following inclusion, while the importance of mental and physical aspects was close at the initial evaluation. This reprioritization effect should reflect a reaction of psychological compensation highlighted by the specific increase of the importance of the mental health dimension over time. The natural evolution of the disease generally includes deterioration and disability. During the second year of follow-up, the order of prioritization was inverted, with the greatest importance given to the physical component. Among the ‘physical-like’ dimensions of the SF-36, we observed a greater importance of physical functioning and role physical dimensions compared to both bodily pain and general health dimensions, for which the scores were relatively stable over time. This finding can be explained by the fact that the disease is not particularly painful and does not affect general health in the short term.
In the not-worsened population, no crossing of the curves was observed during the 24-month follow-up. The mental composite scores had a greater and, consequently, more important impact on the global quality of life index compared to the physical composite scores from the initial evaluation. In this population, the specific analysis of the ‘mental-like’ dimensions indicated that social functioning was clearly an important dimension, showing higher importance indices than the vitality and role emotional dimensions. In contrast, in the worsened population, the three dimensions showed similar importance, reflecting a lower priority for the social domain of the QoL domain. In our study, the lower importance of social life in this group is independent of marital status, although a relationship between the two parameters was previously reported elsewhere, specifically in MS
. Considering marital status as an indirect marker of global social interactions, we thus hypothesize that an MS patient with a severe disease course would anticipate a decrease in his/her social interactions. This reaction would result from the patient’s behavior and beliefs related to the disease. On the contrary, the reprioritization phenomenon found for the social dimension in MS individuals presenting with less severe disease may reflect a willingness to adapt to their situation.
Other methods to detect reprioritization RS have already been developed, specifically in MS populations.
The design-based approaches, specifically the then-test approach
, assess the self-reported patient quality of life at two different times and calculate the difference between the first time (pre-test) and the last time (then-test). Such methods are sensitive and biased and tend to be restricted to retrospective studies
Structural equation modeling (a model-based approach) tests for a change in the magnitude of factor loadings on a common latent variable over time
[30, 31]. This approach cannot always be implemented in studies with small sample sizes because the larger number of parameters to estimate may result in a lack of model convergence. The order in which parameters are tested can affect the conclusion. If a substantial portion of the sample has not undergone an RS, the method is more likely to conclude that the RS did not occur.
More recently, the RS was tested using a recursive partitioning tree analysis that is based on the disease trajectory
[9, 32]. A tree is created for each disease trajectory group. The order of the disability domain indicates reprioritization. This relatively recent data mining method shows promise for identifying small changes in patient-reported outcomes scores over time.
The method based on latent trajectory analysis was centered and used to create trajectories
. An RS was hypothesized to be present when an individual's centered residuals showed a pattern of fluctuation over time. This method does not determine the type of RS that occurred, but it is used to identify subgroups of the population who present an RS.
Finally, methods based on the item response theory should be tested.
The RF method presents several advantages. First, the combination of several trees in a forest results in a stronger classification predictor compared to a single tree. Cross-validation procedures to assess the classification performance of the model are unnecessary because they are already built in, as each tree in the forest has its own training and test (OOB) data. Third, RF are non-parametric, non-linear stable models; no assumptions about the form of underlying relationships between the predictor variables and the response are made
. Fourth, variable importance may be assessed. Finally, the RF algorithm is available in many different open source software packages. Our choice of the RandomForest package
, available as an R implementation of the original RF code
, relied on its wide distribution, ease of use, and the benefit from R data processing functionalities.
The main drawback of our approach is that it only detects the reprioritization component of RS. The role of reprioritization in the score is not quantified. The random forest variable importance measures may be biased in situations where potential predictor variables vary in their scale of measurement or their number of categories
. The method does not provide a statistical test for evaluating the assumption of differences between two importance variable scores, making it difficult to give a clear interpretation when the importance measures are close. A test comparing the score curves should provide an objective decision tool for this purpose.
Strengths and limitations
This study has several strengths and limitations.
The RS phenomenon should not be restricted to RS detection. Future research should be developed to address the remaining essential question: Does the RS need to be integrated into the interpretation of QoL score changes, and how can the weight of the RS in the QoL measure be determined when an RS is detected? The need to restore the usefulness and credibility of the QoL assessment has been recently discussed
[37, 38]; answering this question will contribute to the reintegration of QoL data into clinical practice.
The nature of the use of the QoL questionnaire should be investigated. Some authors expected that disease-specific measures would be less susceptible to a response shift because they query specific symptoms or functional limitations more than generic measures
. We do not accept this assumption because we do not consider MusiQoL to be a symptom-function measure. MusiQoL is a well-validated multidimensional instrument assessing physical, mental, and social domains. Nevertheless, our analyses were performed on an index that is not expected to provide the most sensitive score of changes in the MusiQoL
. This restriction illustrates the results more clearly. Future works should provide data from MusiQoL dimension scores that more accurately demonstrate the RS.
Our study investigated the RS phenomena in the global MusiQoL index. It would be of interest to analyze the RS in the SF36 scores in order to make comparisons of the RS among different diseases.
Another important aspect of this study concerns the appraisal process of the RS, which is not directly measured in the present work. In the absence of an external criterion for the RS (pleasure appraisal processes), an RS interpretation of results will remain disputable
[10, 40]. Future research should measure the RS with direct measures of appraisal.
Future explorations should be performed to compare the capacity of the RF method for detecting the RS with other usual methods and of the degree of convergence of the isolated phenomena.