In psychological and educational measurement, a testlet-based test is a common and popular format, especially in some large-scale assessments. In modeling testlet effects, a standard bifactor model, as a common strategy, assumes different testlet effects and the main effect to be fully independently distributed. However, it is difficult to establish perfectly independent clusters as this assumption. To address this issue, correlations among testlets could be taken into account in fitting data. Moreover, one may desire to maintain a good practical interpretation of the sparse loading matrix. In this paper, we propose data-driven learning of significant correlations in the covariance matrix through a latent variable selection method. Under the proposed method, a regularization is performed on the weak correlations for the extended bifactor model. Further, a stochastic expectation maximization algorithm is employed for efficient computation. Results from simulation studies show the consistency of the proposed method in selecting significant correlations. Empirical data from the 2015 Program for International Student Assessment is analyzed using the proposed method as an example.
Keywords: extended bifactor model; latent variable selection; standard bifactor model; testlet-based test.
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