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For Bayesian hierarchical models, why is there is no analytical expression of the posterior? 


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Bayesian hierarchical models often do not have an analytical expression of the posterior because of the complexity of the data and the model structure. In multi-dimensional problems with non-conjugate priors, evaluating the posterior distribution requires either an analytic or sampling-based approximation, such as Markov chain Monte Carlo (MCMC), Approximate Bayesian computation (ABC), or variational inference . The choice of priors is crucial for posterior propriety and convergence of MCMC samplers . In some cases, the likelihood function is not available explicitly, making it challenging to calculate the posterior . Additionally, in the presence of change points or varying covariates, the selection of the true change point and covariates requires a hierarchical Bayesian methodology and efficient computational strategies like Gibbs sampling . Therefore, the lack of an analytical expression of the posterior in Bayesian hierarchical models is due to the complexity of the models and the need for approximation techniques.

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