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# 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.
#' Prior distributions and options
#'
#' @name priors
#' @description The functions described on this page are used to specify the
#' prior-related arguments of the various modeling functions in the
#' \pkg{rstanarm} package (to view the priors used for an existing model see
#' \code{\link{prior_summary}}).
#'
#' The default priors used in the various \pkg{rstanarm} modeling functions
#' are intended to be \emph{weakly informative} in that they provide moderate
#' regularlization and help stabilize computation. For many applications the
#' defaults will perform well, but prudent use of more informative priors is
#' encouraged. Uniform prior distributions are possible (e.g. by setting
#' \code{\link{stan_glm}}'s \code{prior} argument to \code{NULL}) but, unless
#' the data is very strong, they are not recommended and are \emph{not}
#' non-informative, giving the same probability mass to implausible values as
#' plausible ones.
#'
#' More information on priors is available in the vignette
#' \href{http://mc-stan.org/rstanarm/articles/priors.html}{\emph{Prior
#' Distributions for rstanarm Models}} as well as the vignettes for the
#' various modeling functions. In particular, for details on the
#' priors used for multilevel models see the vignette
#' \href{http://mc-stan.org/rstanarm/articles/glmer.html}{\emph{Estimating
#' Generalized (Non-)Linear Models with Group-Specific Terms with rstanarm}}.
#'
#'
#' @param location Prior location. In most cases, this is the prior mean, but
#' for \code{cauchy} (which is equivalent to \code{student_t} with
#' \code{df=1}), the mean does not exist and \code{location} is the prior
#' median. The default value is \eqn{0}, except for \code{R2} which has no
#' default value for \code{location}. For \code{R2}, \code{location} pertains
#' to the prior location of the \eqn{R^2} under a Beta distribution, but the
#' interpretation of the \code{location} parameter depends on the specified
#' value of the \code{what} argument (see the \emph{R2 family} section in
#' \strong{Details}).
#' @param scale Prior scale. The default depends on the family (see
#' \strong{Details}).
#' @param df,df1,df2 Prior degrees of freedom. The default is \eqn{1} for
#' \code{student_t}, in which case it is equivalent to \code{cauchy}. For the
#' hierarchical shrinkage priors (\code{hs} and \code{hs_plus}) the degrees of
#' freedom parameter(s) default to \eqn{1}. For the \code{product_normal}
#' prior, the degrees of freedom parameter must be an integer (vector) that is
#' at least \eqn{2} (the default).
#' @param global_df,global_scale,slab_df,slab_scale Optional arguments for the
#' hierarchical shrinkage priors. See the \emph{Hierarchical shrinkage family}
#' section below.
#' @param what A character string among \code{'mode'} (the default),
#' \code{'mean'}, \code{'median'}, or \code{'log'} indicating how the
#' \code{location} parameter is interpreted in the \code{LKJ} case. If
#' \code{'log'}, then \code{location} is interpreted as the expected
#' logarithm of the \eqn{R^2} under a Beta distribution. Otherwise,
#' \code{location} is interpreted as the \code{what} of the \eqn{R^2}
#' under a Beta distribution. If the number of predictors is less than
#' or equal to two, the mode of this Beta distribution does not exist
#' and an error will prompt the user to specify another choice for
#' \code{what}.
#' @param autoscale A logical scalar, defaulting to \code{TRUE}. If \code{TRUE}
#' then the scales of the priors on the intercept and regression coefficients
#' may be additionally modified internally by \pkg{rstanarm} in the following
#' cases. First, for Gaussian models only, the prior scales for the intercept,
#' coefficients, and the auxiliary parameter \code{sigma} (error standard
#' deviation) are multiplied by \code{sd(y)}. Additionally --- not only for
#' Gaussian models --- if the \code{QR} argument to the model fitting function
#' (e.g. \code{stan_glm}) is \code{FALSE} then: for a predictor with only one
#' value nothing is changed; for a predictor \code{x} with exactly two unique
#' values, we take the user-specified (or default) scale(s) for the selected
#' priors and divide by the range of \code{x}; for a predictor \code{x} with
#' more than two unique values, we divide the prior scale(s) by \code{sd(x)}.
#'
#' @details The details depend on the family of the prior being used:
#' \subsection{Student t family}{
#' Family members:
#' \itemize{
#' \item \code{normal(location, scale)}
#' \item \code{student_t(df, location, scale)}
#' \item \code{cauchy(location, scale)}
#' }
#' Each of these functions also takes an argument \code{autoscale}.
#'
#' For the prior distribution for the intercept, \code{location},
#' \code{scale}, and \code{df} should be scalars. For the prior for the other
#' coefficients they can either be vectors of length equal to the number of
#' coefficients (not including the intercept), or they can be scalars, in
#' which case they will be recycled to the appropriate length. As the
#' degrees of freedom approaches infinity, the Student t distribution
#' approaches the normal distribution and if the degrees of freedom are one,
#' then the Student t distribution is the Cauchy distribution.
#'
#' If \code{scale} is not specified it will default to \eqn{10} for the
#' intercept and \eqn{2.5} for the other coefficients, unless the probit link
#' function is used, in which case these defaults are scaled by a factor of
#' \code{dnorm(0)/dlogis(0)}, which is roughly \eqn{1.6}.
#'
#' If the \code{autoscale} argument is \code{TRUE} (the default), then the
#' scales will be further adjusted as described above in the documentation of
#' the \code{autoscale} argument in the \strong{Arguments} section.
#' }
#' \subsection{Hierarchical shrinkage family}{
#' Family members:
#' \itemize{
#' \item \code{hs(df, global_df, global_scale, slab_df, slab_scale)}
#' \item \code{hs_plus(df1, df2, global_df, global_scale, slab_df, slab_scale)}
#' }
#'
#' The hierarchical shrinkage priors are normal with a mean of zero and a
#' standard deviation that is also a random variable. The traditional
#' hierarchical shrinkage prior utilizes a standard deviation that is
#' distributed half Cauchy with a median of zero and a scale parameter that is
#' also half Cauchy. This is called the "horseshoe prior". The hierarchical
#' shrinkage (\code{hs}) prior in the \pkg{rstanarm} package instead utilizes
#' a regularized horseshoe prior, as described by Piironen and Vehtari (2017),
#' which recommends setting the \code{global_scale} argument equal to the ratio
#' of the expected number of non-zero coefficients to the expected number of
#' zero coefficients, divided by the square root of the number of observations.
#'
#' The hierarhical shrinkpage plus (\code{hs_plus}) prior is similar except
#' that the standard deviation that is distributed as the product of two
#' independent half Cauchy parameters that are each scaled in a similar way
#' to the \code{hs} prior.
#'
#' The hierarchical shrinkage priors have very tall modes and very fat tails.
#' Consequently, they tend to produce posterior distributions that are very
#' concentrated near zero, unless the predictor has a strong influence on the
#' outcome, in which case the prior has little influence. Hierarchical
#' shrinkage priors often require you to increase the
#' \code{\link{adapt_delta}} tuning parameter in order to diminish the number
#' of divergent transitions. For more details on tuning parameters and
#' divergent transitions see the Troubleshooting section of the \emph{How to
#' Use the rstanarm Package} vignette.
#' }
#' \subsection{Laplace family}{
#' Family members:
#' \itemize{
#' \item \code{laplace(location, scale)}
#' \item \code{lasso(df, location, scale)}
#' }
#' Each of these functions also takes an argument \code{autoscale}.
#'
#' The Laplace distribution is also known as the double-exponential
#' distribution. It is a symmetric distribution with a sharp peak at its mean
#' / median / mode and fairly long tails. This distribution can be motivated
#' as a scale mixture of normal distributions and the remarks above about the
#' normal distribution apply here as well.
#'
#' The lasso approach to supervised learning can be expressed as finding the
#' posterior mode when the likelihood is Gaussian and the priors on the
#' coefficients have independent Laplace distributions. It is commonplace in
#' supervised learning to choose the tuning parameter by cross-validation,
#' whereas a more Bayesian approach would be to place a prior on \dQuote{it},
#' or rather its reciprocal in our case (i.e. \emph{smaller} values correspond
#' to more shrinkage toward the prior location vector). We use a chi-square
#' prior with degrees of freedom equal to that specified in the call to
#' \code{lasso} or, by default, 1. The expectation of a chi-square random
#' variable is equal to this degrees of freedom and the mode is equal to the
#' degrees of freedom minus 2, if this difference is positive.
#'
#' It is also common in supervised learning to standardize the predictors
#' before training the model. We do not recommend doing so. Instead, it is
#' better to specify \code{autoscale = TRUE} (the default value), which
#' will adjust the scales of the priors according to the dispersion in the
#' variables. See the documentation of the \code{autoscale} argument above
#' and also the \code{\link{prior_summary}} page for more information.
#' }
#' \subsection{Product-normal family}{
#' Family members:
#' \itemize{
#' \item \code{product_normal(df, location, scale)}
#' }
#' The product-normal distribution is the product of at least two independent
#' normal variates each with mean zero, shifted by the \code{location}
#' parameter. It can be shown that the density of a product-normal variate is
#' symmetric and infinite at \code{location}, so this prior resembles a
#' \dQuote{spike-and-slab} prior for sufficiently large values of the
#' \code{scale} parameter. For better or for worse, this prior may be
#' appropriate when it is strongly believed (by someone) that a regression
#' coefficient \dQuote{is} equal to the \code{location}, parameter even though
#' no true Bayesian would specify such a prior.
#'
#' Each element of \code{df} must be an integer of at least \eqn{2} because
#' these \dQuote{degrees of freedom} are interpreted as the number of normal
#' variates being multiplied and then shifted by \code{location} to yield the
#' regression coefficient. Higher degrees of freedom produce a sharper
#' spike at \code{location}.
#'
#' Each element of \code{scale} must be a non-negative real number that is
#' interpreted as the standard deviation of the normal variates being
#' multiplied and then shifted by \code{location} to yield the regression
#' coefficient. In other words, the elements of \code{scale} may differ, but
#' the k-th standard deviation is presumed to hold for all the normal deviates
#' that are multiplied together and shifted by the k-th element of
#' \code{location} to yield the k-th regression coefficient. The elements of
#' \code{scale} are not the prior standard deviations of the regression
#' coefficients. The prior variance of the regression coefficients is equal to
#' the scale raised to the power of \eqn{2} times the corresponding element of
#' \code{df}. Thus, larger values of \code{scale} put more prior volume on
#' values of the regression coefficient that are far from zero.
#' }
#' \subsection{Dirichlet family}{
#' Family members:
#' \itemize{
#' \item \code{dirichlet(concentration)}
#' }
#'
#' The Dirichlet distribution is a multivariate generalization of the beta
#' distribution. It is perhaps the easiest prior distribution to specify
#' because the concentration parameters can be interpreted as prior counts
#' (although they need not be integers) of a multinomial random variable.
#'
#' The Dirichlet distribution is used in \code{\link{stan_polr}} for an
#' implicit prior on the cutpoints in an ordinal regression model. More
#' specifically, the Dirichlet prior pertains to the prior probability of
#' observing each category of the ordinal outcome when the predictors are at
#' their sample means. Given these prior probabilities, it is straightforward
#' to add them to form cumulative probabilities and then use an inverse CDF
#' transformation of the cumulative probabilities to define the cutpoints.
#'
#' If a scalar is passed to the \code{concentration} argument of the
#' \code{dirichlet} function, then it is replicated to the appropriate length
#' and the Dirichlet distribution is symmetric. If \code{concentration} is a
#' vector and all elements are \eqn{1}, then the Dirichlet distribution is
#' jointly uniform. If all concentration parameters are equal but greater than
#' \eqn{1} then the prior mode is that the categories are equiprobable, and
#' the larger the value of the identical concentration parameters, the more
#' sharply peaked the distribution is at the mode. The elements in
#' \code{concentration} can also be given different values to represent that
#' not all outcome categories are a priori equiprobable.
#' }
#' \subsection{Covariance matrices}{
#' Family members:
#' \itemize{
#' \item \code{decov(regularization, concentration, shape, scale)}
#' \item \code{lkj(regularization, scale, df)}
#' }
#' (Also see vignette for \code{stan_glmer},
#' \href{http://mc-stan.org/rstanarm/articles/glmer.html}{\emph{Estimating
#' Generalized (Non-)Linear Models with Group-Specific Terms with rstanarm}})
#'
#' Covariance matrices are decomposed into correlation matrices and
#' variances. The variances are in turn decomposed into the product of a
#' simplex vector and the trace of the matrix. Finally, the trace is the
#' product of the order of the matrix and the square of a scale parameter.
#' This prior on a covariance matrix is represented by the \code{decov}
#' function.
#'
#' The prior for a correlation matrix is called LKJ whose density is
#' proportional to the determinant of the correlation matrix raised to the
#' power of a positive regularization parameter minus one. If
#' \code{regularization = 1} (the default), then this prior is jointly
#' uniform over all correlation matrices of that size. If
#' \code{regularization > 1}, then the identity matrix is the mode and in the
#' unlikely case that \code{regularization < 1}, the identity matrix is the
#' trough.
#'
#' The trace of a covariance matrix is equal to the sum of the variances. We
#' set the trace equal to the product of the order of the covariance matrix
#' and the \emph{square} of a positive scale parameter. The particular
#' variances are set equal to the product of a simplex vector --- which is
#' non-negative and sums to \eqn{1} --- and the scalar trace. In other words,
#' each element of the simplex vector represents the proportion of the trace
#' attributable to the corresponding variable.
#'
#' A symmetric Dirichlet prior is used for the simplex vector, which has a
#' single (positive) \code{concentration} parameter, which defaults to
#' \eqn{1} and implies that the prior is jointly uniform over the space of
#' simplex vectors of that size. If \code{concentration > 1}, then the prior
#' mode corresponds to all variables having the same (proportion of total)
#' variance, which can be used to ensure the the posterior variances are not
#' zero. As the \code{concentration} parameter approaches infinity, this
#' mode becomes more pronounced. In the unlikely case that
#' \code{concentration < 1}, the variances are more polarized.
#'
#' If all the variables were multiplied by a number, the trace of their
#' covariance matrix would increase by that number squared. Thus, it is
#' reasonable to use a scale-invariant prior distribution for the positive
#' scale parameter, and in this case we utilize a Gamma distribution, whose
#' \code{shape} and \code{scale} are both \eqn{1} by default, implying a
#' unit-exponential distribution. Set the \code{shape} hyperparameter to some
#' value greater than \eqn{1} to ensure that the posterior trace is not zero.
#'
#' If \code{regularization}, \code{concentration}, \code{shape} and / or
#' \code{scale} are positive scalars, then they are recycled to the
#' appropriate length. Otherwise, each can be a positive vector of the
#' appropriate length, but the appropriate length depends on the number of
#' covariance matrices in the model and their sizes. A one-by-one covariance
#' matrix is just a variance and thus does not have \code{regularization} or
#' \code{concentration} parameters, but does have \code{shape} and
#' \code{scale} parameters for the prior standard deviation of that
#' variable.
#'
#' Note that for \code{\link{stan_mvmer}} and \code{\link{stan_jm}} models an
#' additional prior distribution is provided through the \code{lkj} function.
#' This prior is in fact currently used as the default for those modelling
#' functions (although \code{decov} is still available as an option if the user
#' wishes to specify it through the \code{prior_covariance} argument). The
#' \code{lkj} prior uses the same decomposition of the covariance matrices
#' into correlation matrices and variances, however, the variances are not
#' further decomposed into a simplex vector and the trace; instead the
#' standard deviations (square root of the variances) for each of the group
#' specific parameters are given a half Student t distribution with the
#' scale and df parameters specified through the \code{scale} and \code{df}
#' arguments to the \code{lkj} function. The scale parameter default is 10
#' which is then autoscaled, whilst the df parameter default is 1
#' (therefore equivalent to a half Cauchy prior distribution for the
#' standard deviation of each group specific parameter). This prior generally
#' leads to similar results as the \code{decov} prior, but it is also likely
#' to be **less** diffuse compared with the \code{decov} prior; therefore it
#' sometimes seems to lead to faster estimation times, hence why it has
#' been chosen as the default prior for \code{\link{stan_mvmer}} and
#' \code{\link{stan_jm}} where estimation times can be long.
#' }
#' \subsection{R2 family}{
#' Family members:
#' \itemize{
#' \item \code{R2(location, what)}
#' }
#'
#' The \code{\link{stan_lm}}, \code{\link{stan_aov}}, and
#' \code{\link{stan_polr}} functions allow the user to utilize a function
#' called \code{R2} to convey prior information about all the parameters.
#' This prior hinges on prior beliefs about the location of \eqn{R^2}, the
#' proportion of variance in the outcome attributable to the predictors,
#' which has a \code{\link[stats]{Beta}} prior with first shape
#' hyperparameter equal to half the number of predictors and second shape
#' hyperparameter free. By specifying \code{what} to be the prior mode (the
#' default), mean, median, or expected log of \eqn{R^2}, the second shape
#' parameter for this Beta distribution is determined internally. If
#' \code{what = 'log'}, location should be a negative scalar; otherwise it
#' should be a scalar on the \eqn{(0,1)} interval.
#'
#' For example, if \eqn{R^2 = 0.5}, then the mode, mean, and median of
#' the \code{\link[stats]{Beta}} distribution are all the same and thus the
#' second shape parameter is also equal to half the number of predictors.
#' The second shape parameter of the \code{\link[stats]{Beta}} distribution
#' is actually the same as the shape parameter in the LKJ prior for a
#' correlation matrix described in the previous subsection. Thus, the smaller
#' is \eqn{R^2}, the larger is the shape parameter, the smaller are the
#' prior correlations among the outcome and predictor variables, and the more
#' concentrated near zero is the prior density for the regression
#' coefficients. Hence, the prior on the coefficients is regularizing and
#' should yield a posterior distribution with good out-of-sample predictions
#' \emph{if} the prior location of \eqn{R^2} is specified in a reasonable
#' fashion.
#' }
#' @return A named list to be used internally by the \pkg{rstanarm} model
#' fitting functions.
#' @seealso The various vignettes for the \pkg{rstanarm} package also discuss
#' and demonstrate the use of some of the supported prior distributions.
#'
#' @templateVar bdaRef \url{http://stat.columbia.edu/~gelman/book/}
#' @template reference-bda
#'
#' @references
#' Gelman, A., Jakulin, A., Pittau, M. G., and Su, Y. (2008). A weakly
#' informative default prior distribution for logistic and other regression
#' models. \emph{Annals of Applied Statistics}. 2(4), 1360--1383.
#'
#' @template reference-piironen-vehtari
#' @template reference-stan-manual
#'
#' @examples
#' fmla <- mpg ~ wt + qsec + drat + am
#'
#' # Draw from prior predictive distribution (by setting prior_PD = TRUE)
#' prior_pred_fit <- stan_glm(fmla, data = mtcars, prior_PD = TRUE,
#' chains = 1, seed = 12345, iter = 250, # for speed only
#' prior = student_t(df = 4, 0, 2.5),
#' prior_intercept = cauchy(0,10),
#' prior_aux = exponential(1/2))
#' plot(prior_pred_fit, "hist")
#'
#' \donttest{
#' # Can assign priors to names
#' N05 <- normal(0, 5)
#' fit <- stan_glm(fmla, data = mtcars, prior = N05, prior_intercept = N05)
#' }
#'
#' # Visually compare normal, student_t, cauchy, laplace, and product_normal
#' compare_priors <- function(scale = 1, df_t = 2, xlim = c(-10, 10)) {
#' dt_loc_scale <- function(x, df, location, scale) {
#' 1/scale * dt((x - location)/scale, df)
#' }
#' dlaplace <- function(x, location, scale) {
#' 0.5 / scale * exp(-abs(x - location) / scale)
#' }
#' dproduct_normal <- function(x, scale) {
#' besselK(abs(x) / scale ^ 2, nu = 0) / (scale ^ 2 * pi)
#' }
#' stat_dist <- function(dist, ...) {
#' ggplot2::stat_function(ggplot2::aes_(color = dist), ...)
#' }
#' ggplot2::ggplot(data.frame(x = xlim), ggplot2::aes(x)) +
#' stat_dist("normal", size = .75, fun = dnorm,
#' args = list(mean = 0, sd = scale)) +
#' stat_dist("student_t", size = .75, fun = dt_loc_scale,
#' args = list(df = df_t, location = 0, scale = scale)) +
#' stat_dist("cauchy", size = .75, linetype = 2, fun = dcauchy,
#' args = list(location = 0, scale = scale)) +
#' stat_dist("laplace", size = .75, linetype = 2, fun = dlaplace,
#' args = list(location = 0, scale = scale)) +
#' stat_dist("product_normal", size = .75, linetype = 2, fun = dproduct_normal,
#' args = list(scale = 1))
#' }
#' # Cauchy has fattest tails, followed by student_t, laplace, and normal
#' compare_priors()
#'
#' # The student_t with df = 1 is the same as the cauchy
#' compare_priors(df_t = 1)
#'
#' # Even a scale of 5 is somewhat large. It gives plausibility to rather
#' # extreme values
#' compare_priors(scale = 5, xlim = c(-20,20))
#'
#' # If you use a prior like normal(0, 1000) to be "non-informative" you are
#' # actually saying that a coefficient value of e.g. -500 is quite plausible
#' compare_priors(scale = 1000, xlim = c(-1000,1000))
#'
NULL
#' @rdname priors
#' @export
normal <- function(location = 0, scale = NULL, autoscale = TRUE) {
validate_parameter_value(scale)
nlist(dist = "normal", df = NA, location, scale, autoscale)
}
#' @rdname priors
#' @export
student_t <- function(df = 1, location = 0, scale = NULL, autoscale = TRUE) {
validate_parameter_value(scale)
validate_parameter_value(df)
nlist(dist = "t", df, location, scale, autoscale)
}
#' @rdname priors
#' @export
cauchy <- function(location = 0, scale = NULL, autoscale = TRUE) {
student_t(df = 1, location = location, scale = scale, autoscale)
}
#' @rdname priors
#' @export
hs <- function(df = 1, global_df = 1, global_scale = 0.01,
slab_df = 4, slab_scale = 2.5) {
validate_parameter_value(df)
validate_parameter_value(global_df)
validate_parameter_value(global_scale)
validate_parameter_value(slab_df)
validate_parameter_value(slab_scale)
nlist(dist = "hs", df, location = 0, scale = 1,
global_df, global_scale, slab_df, slab_scale)
}
#' @rdname priors
#' @export
hs_plus <- function(df1 = 1, df2 = 1, global_df = 1, global_scale = 0.01,
slab_df = 4, slab_scale = 2.5) {
validate_parameter_value(df1)
validate_parameter_value(df2)
validate_parameter_value(global_df)
validate_parameter_value(global_scale)
validate_parameter_value(slab_df)
validate_parameter_value(slab_scale)
# scale gets used as a second df hyperparameter
nlist(dist = "hs_plus", df = df1, location = 0, scale = df2, global_df,
global_scale, slab_df, slab_scale)
}
#' @rdname priors
#' @export
laplace <- function(location = 0, scale = NULL, autoscale = TRUE) {
nlist(dist = "laplace", df = NA, location, scale, autoscale)
}
#' @rdname priors
#' @export
lasso <- function(df = 1, location = 0, scale = NULL, autoscale = TRUE) {
nlist(dist = "lasso", df, location, scale, autoscale)
}
#' @rdname priors
#' @export
product_normal <- function(df = 2, location = 0, scale = 1) {
validate_parameter_value(df)
stopifnot(all(df >= 1), all(df == as.integer(df)))
validate_parameter_value(scale)
nlist(dist = "product_normal", df, location, scale)
}
#' @rdname priors
#' @export
#' @param rate Prior rate for the exponential distribution. Defaults to
#' \code{1}. For the exponential distribution, the rate parameter is the
#' \emph{reciprocal} of the mean.
#'
exponential <- function(rate = 1, autoscale = TRUE) {
stopifnot(length(rate) == 1)
validate_parameter_value(rate)
nlist(dist = "exponential",
df = NA, location = NA, scale = 1/rate,
autoscale)
}
#' @rdname priors
#' @export
#' @param regularization Exponent for an LKJ prior on the correlation matrix in
#' the \code{decov} or \code{lkj} prior. The default is \eqn{1}, implying a
#' joint uniform prior.
#' @param concentration Concentration parameter for a symmetric Dirichlet
#' distribution. The default is \eqn{1}, implying a joint uniform prior.
#' @param shape Shape parameter for a gamma prior on the scale parameter in the
#' \code{decov} prior. If \code{shape} and \code{scale} are both \eqn{1} (the
#' default) then the gamma prior simplifies to the unit-exponential
#' distribution.
decov <- function(regularization = 1, concentration = 1,
shape = 1, scale = 1) {
validate_parameter_value(regularization)
validate_parameter_value(concentration)
validate_parameter_value(shape)
validate_parameter_value(scale)
nlist(dist = "decov", regularization, concentration, shape, scale)
}
#' @rdname priors
#' @export
lkj <- function(regularization = 1, scale = 10, df = 1, autoscale = TRUE) {
validate_parameter_value(regularization)
validate_parameter_value(scale)
validate_parameter_value(df)
nlist(dist = "lkj", regularization, scale, df, autoscale)
}
#' @rdname priors
#' @export
dirichlet <- function(concentration = 1) {
validate_parameter_value(concentration)
nlist(dist = "dirichlet", concentration)
}
#' @rdname priors
#' @export
R2 <- function(location = NULL, what = c("mode", "mean", "median", "log")) {
what <- match.arg(what)
validate_R2_location(location, what)
list(dist = "R2", location = location, what = what, df = 0, scale = 0)
}
# internal ----------------------------------------------------------------
# Check for positive scale or df parameter (NULL ok)
#
# @param x The value to check.
# @return Either an error is thrown or \code{TRUE} is returned invisibly.
validate_parameter_value <- function(x) {
nm <- deparse(substitute(x))
if (!is.null(x)) {
if (!is.numeric(x))
stop(nm, " should be NULL or numeric", call. = FALSE)
if (any(x <= 0))
stop(nm, " should be positive", call. = FALSE)
}
invisible(TRUE)
}
# Throw informative error if 'location' isn't valid for the particular 'what'
# specified or isn't the right length.
#
# @param location,what User's location and what arguments to R2()
# @return Either an error is thrown or TRUE is returned invisibly.
#
validate_R2_location <- function(location = NULL, what) {
stopifnot(is.numeric(location))
if (length(location) > 1)
stop(
"The 'R2' function only accepts a single value for 'location', ",
"which applies to the prior R^2. ",
"If you are trying to put different priors on different coefficients ",
"rather than specify a joint prior via 'R2', you can use stan_glm ",
"which accepts a wider variety of priors, many of which allow ",
"specifying arguments as vectors.",
call. = FALSE
)
if (what == "log") {
if (location >= 0)
stop("If 'what' is 'log' then location must be negative.", call. = FALSE)
} else if (what == "mode") {
if (location <= 0 || location > 1)
stop("If 'what' is 'mode', location must be in (0,1].",
call. = FALSE)
} else { # "mean", "median"
if (location <= 0 || location >= 1)
stop("If 'what' is 'mean' or 'median', location must be in (0,1).",
call. = FALSE)
}
invisible(TRUE)
}
# For the R2 prior, calculate LKJ shape eta
#
# @param location,what User's R2 prior arguments.
# @param K number of predictors.
# @return A positive scalar.
#
make_eta <- function(location, what = c("mode", "mean", "median", "log"), K) {
stopifnot(length(location) == 1, is.numeric(location))
stopifnot(is.numeric(K), K == as.integer(K))
if (K == 0)
stop("R2 prior is not applicable when there are no covariates.",
call. = FALSE)
what <- match.arg(what)
half_K <- K / 2
if (what == "mode") {
stopifnot(location > 0, location <= 1)
if (K <= 2)
stop(paste("R2 prior error.",
"The mode of the beta distribution does not exist",
"with fewer than three predictors.",
"Specify 'what' as 'mean', 'median', or 'log' instead."),
call. = FALSE)
eta <- (half_K - 1 - location * half_K + location * 2) / location
} else if (what == "mean") {
stopifnot(location > 0, location < 1)
eta <- (half_K - location * half_K) / location
} else if (what == "median") {
stopifnot(location > 0, location < 1)
FUN <- function(eta) qbeta(0.5, half_K, qexp(eta)) - location
eta <- qexp(uniroot(FUN, interval = 0:1)$root)
} else { # what == "log"
stopifnot(location < 0)
FUN <- function(eta) digamma(half_K) - digamma(half_K + qexp(eta)) - location
eta <- qexp(uniroot(FUN, interval = 0:1,
f.lower = -location,
f.upper = -.Machine$double.xmax)$root)
}
return(eta)
}
```

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