A character. String specifying type of parameters to set ('flat', 'prior_mean', 'posterior_mean', 'prior_draw', 'posterior_draw', 'define'). With param_type set to define use arguments to be passed to make_priors; otherwise flat sets equal probabilities on each nodal param_type in each parameter set; prior_mean, prior_draw, posterior_mean, posterior_draw take parameters as the means or as draws from the prior or posterior.