ra model parameter, we need to set a prior distribution for it. A convenient choice is the conjugate inverse chi-squared distribution, which allows Gibbs sampling step for sigma2 after every Metropolis-Hastings update for the other parameters. This is acchieved by <tt>options.updatesigma=1</tt>, below. The default prior is uninformative, but we can set the prior parameters with the following options. Option <tt>model.N</tt> for the number of observatios is needed for 'updatesigma', if it is not given, the c