# Part of the rstanarm package for estimating model parameters
# Copyright (C) 2015, 2016, 2017 Trustees of Columbia University
#
# This program is free software; you can redistribute it and/or
# modify it under the terms of the GNU General Public License
# as published by the Free Software Foundation; either version 3
# of the License, or (at your option) any later version.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
# GNU General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with this program; if not, write to the Free Software
# Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
#' \code{adapt_delta}: Target average acceptance probability
#'
#' Details about the \code{adapt_delta} argument to \pkg{rstanarm}'s modeling
#' functions.
#'
#' @name adapt_delta
#' @template reference-stan-manual
#' @references Brief Guide to Stan's Warnings:
#' \url{https://mc-stan.org/misc/warnings.html#divergent-transitions-after-warmup}
#'
#'
#' @details For the No-U-Turn Sampler (NUTS), the variant of Hamiltonian Monte
#' Carlo used used by \pkg{rstanarm}, \code{adapt_delta} is the target average
#' proposal acceptance probability during Stan's adaptation period.
#' \code{adapt_delta} is ignored by \pkg{rstanarm} if the \code{algorithm} argument
#' is not set to \code{"sampling"}.
#'
#' The default value of \code{adapt_delta} is 0.95, except when the prior for
#' the regression coefficients is \code{\link{R2}}, \code{\link{hs}}, or
#' \code{\link{hs_plus}}, in which case the default is 0.99.
#'
#' These defaults are higher (more conservative) than the default of
#' \code{adapt_delta=0.8} used in the \pkg{rstan} package, which may result in
#' slower sampling speeds but will be more robust to posterior distributions
#' with high curvature.
#'
#' In general you should not need to change \code{adapt_delta} unless you see
#' a warning message about divergent transitions, in which case you can
#' increase \code{adapt_delta} from the default to a value \emph{closer} to 1
#' (e.g. from 0.95 to 0.99, or from 0.99 to 0.999, etc). The step size used by
#' the numerical integrator is a function of \code{adapt_delta} in that
#' increasing \code{adapt_delta} will result in a smaller step size and fewer
#' divergences. Increasing \code{adapt_delta} will typically result in a
#' slower sampler, but it will always lead to a more robust sampler.
NULL
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