A single-level random-effects cross-lagged panel model for longitudinal mediation analysis

Cross-lagged panel models (CLPMs) are widely used to test mediation with longitudinal panel data. One major limitation of the CLPMs is that the model effects are assumed to be fixed across individuals. This assumption is likely to be violated (i.e., the model effects are random across individuals) in practice. When this happens, the CLPMs can potentially yield biased parameter estimates and misleading statistical inferences. This article proposes a model named a random-effects cross-lagged panel model (RE-CLPM) to account for random effects in CLPMs. Simulation studies show that the RE-CLPM outperforms the CLPM in recovering the mean indirect and direct effects in a longitudinal mediation analysis when random effects exist in the population. The performance of the RE-CLPM is robust to a certain degree, even when the random effects are not normally distributed. In addition, the RE-CLPM does not produce harmful results when the model effects are in fact fixed in the population. Implications of the simulation studies and potential directions for future research are discussed

Testing Measurement Invariance with Ordinal Missing Data: A Comparison of Estimators and Missing Data Techniques

Ordinal missing data are common in measurement equivalence/invariance (ME/I) testing studies. However, there is a lack of guidance on the appropriate method to deal with ordinal missing data in ME/I testing. Five methods may be used to deal with ordinal missing data in ME/I testing, including the continuous full information maximum likelihood estimation method (FIML), continuous robust FIML (rFIML), FIML with probit links (pFIML), FIML with logit links (lFIML), and mean and variance adjusted weight least squared estimation method combined with pairwise deletion (WLSMV_PD). The current study evaluates the relative performance of these methods in producing valid chi-square difference tests (Δχ2) and accurate parameter estimates. The result suggests that all methods except for WLSMV_PD can reasonably control the type I error rates of (Δχ2) tests and maintain sufficient power to detect noninvariance in most conditions. Only pFIML and lFIML yield accurate factor loading estimates and standard errors across all the conditions. Recommendations are provided to researchers based on the results

Strategies to deal with ordinal missing data for measurement invariance testing and specification searches – A comparison of commonly used methods

Measurement equivalent/invariance is a key concept in psychological testing. Failing to correctly identify non-invariant items can lead biased group comparisons and biased selections. The methodological literature on measurement equivalent/invariance (ME/I) and specification searches in structural equation modeling (SEM) usually consider only complete data. In practice, ME/I tests are often done on Likert scales which involve ordinal variables. Missing data on ordinal variables can be problematic in ME/I tests based on the chi-square statistic ( ) and modification indices. To deal with missing ordinal data, a recommended strategy is to combine multiple imputation with weighted least squares estimation methods. However, both statistic and modification indices are not available with this strategy. Consequently, researchers have to adopt “suboptimal” methods: 1) use full information maximum likelihood (FIML) by treating ordinal data as normally distributed continuous data; 2) use robust FIML by treating ordinal data as non-normally distributed continuous data; and, 3) use weighted least squares (WLSMV) estimators with suboptimal missing data handling techniques, such as pairwise deletion. Previous studies have found that any of the strategies may bias the point estimates and statistics in SEM. Yet, there has been no systematic comparison of the suboptimal strategies, especially in the context of ME/I tests or chi-square difference tests (Δ tests). Thus, the goals of my dissertation are to investigate the relative performance of these commonly used suboptimal strategies on the Δ tests and modification indices in ME/I testing with ordinal missing data. Two simulation studies were conducted. Study 1 aimed to compare the three strategies in terms of the accuracy and efficiency of parameter estimates as well as the type I error rate and power of Δ tests. Study 2 aimed to examine the relative performance of the strategies on specification search. I investigated three backward specification search methods based on the largest modification index using the three suboptimal methods described above and compared it to a recently proposed forward specification search method based on confidence intervals (CI approach), which can be implemented in the “optimal” approach of WLSMV using multiple imputations. The first simulation study showed that when the target data set contains a substantive amount of ordinal missing data, using the Δ tests and modification indices obtained from WLSMV with pairwise deletion lead to a substantive inflation of type I error rates. In contrast, the Δ tests and modification indices obtained from FIML approaches had a better ability to control the type error with sufficient power to test measurement invariance under most conditions. However, parameter estimates were biased for the FIML approaches. In the second simulation study, FIML based modification indices could identify more effectively the correct invariant factor loadings than the modification indices from the WLSMV estimator using pairwise deletion or the CI approach from the WLSMV estimator with multiple imputations. However, all search methods showed an inflated type I error at the model level because none of the methods could effectively locate non-invariant thresholds. Future directions of the ordinal missing data in invariance testing are discussed and practical suggestions for empirical researchers are provided

Expression Profiling in the Muscular Dystrophies: Identification of Novel Aspects of Molecular Pathophysiology

We used expression profiling to define the pathophysiological cascades involved in the progression of two muscular dystrophies with known primary biochemical defects, dystrophin deficiency (Duchenne muscular dystrophy) and α-sarcoglycan deficiency (a dystrophin-associated protein). We employed a novel protocol for expression profiling in human tissues using mixed samples of multiple patients and iterative comparisons of duplicate datasets. We found evidence for both incomplete differentiation of patient muscle, and for dedifferentiation of myofibers to alternative lineages with advancing age. One developmentally regulated gene characterized in detail, α-cardiac actin, showed abnormal persistent expression after birth in 60% of Duchenne dystrophy myofibers. The majority of myofibers (∼80%) remained strongly positive for this protein throughout the course of the disease. Other developmentally regulated genes that showed widespread overexpression in these muscular dystrophies included embryonic myosin heavy chain, versican, acetylcholine receptor α-1, secreted protein, acidic and rich in cysteine/osteonectin, and thrombospondin 4. We hypothesize that the abnormal Ca2+ influx in dystrophin- and α-sarcoglycan–deficient myofibers leads to altered developmental programming of developing and regenerating myofibers. The finding of upregulation of HLA-DR and factor XIIIa led to the novel identification of activated dendritic cell infiltration in dystrophic muscle; these cells mediate immune responses and likely induce microenvironmental changes in muscle. We also document a general metabolic crisis in dystrophic muscle, with large scale downregulation of nuclear-encoded mitochondrial gene expression. Finally, our expression profiling results show that primary genetic defects can be identified by a reduction in the corresponding RNA

Minimally invasive strategy for gynecologic cancer with solitary periacetabular metastasis

SummaryTumor with bone metastases to the periacetabulum is rare, and its surgical management is challenging. Instead of wide excision with reconstruction of the hip joint, we used a relatively noninvasive method to manage periacetabular metastasis. Such a procedure for this condition has the benefits of short surgical time, less bleeding, and fewer complications during surgery. Our surgical management of the case reported here included curettage, phenol cauterization and filling of cisplatin-loaded cement in order to reduce local recurrence. After following-up for 2 years, there was no local recurrence and disease progression