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multivariate poisson hidden markov models for analysis of spatial ...

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eplications are generally sufficient <strong>for</strong> the standard error estimation (Efron and<br />

Tibshirani, 1993).<br />

On the identifiability <strong>of</strong> a mixture model, if the component densities <strong>of</strong> the mixture<br />

belong to the same parametric family, then the likelihood does not change under a<br />

permutation <strong>of</strong> the component labels in the parameter Φ and hence neither does its<br />

maximum likelihood estimate Φ . This raises the question <strong>of</strong> whether the so-called<br />

label-switching problem (<strong>for</strong> example, what you have as the first cluster now will be the<br />

second cluster in the next sample and so on) occurs in the generation <strong>of</strong> the bootstrap<br />

replications <strong>of</strong> the maximum likelihood estimation, as in Monte Carlo Markov chain<br />

computations involving mixture <strong>models</strong>. McLachlan and Peel (2000) explained that<br />

according to their experience it has not arisen, as they always take the maximum<br />

likelihood estimate Φ <br />

calculated from the original data to be the initial value <strong>of</strong><br />

parameter in applying the EM algorithm to each bootstrap sample.<br />

The following steps were used to calculate the bootstrapped standard errors <strong>for</strong> both<br />

<strong>models</strong>:<br />

(a) Multivariate Poisson finite mixture model and<br />

(b) Multivariate Poisson <strong>hidden</strong> Markov model.<br />

Step 1: Using estimated means and transition probabilities/or mixing proportions from<br />

different states/or components simulate the mixture distribution <strong>of</strong> data.<br />

101

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