# stan_glm

##### Bayesian generalized linear models via Stan

Generalized linear modeling with optional prior distributions for the coefficients, intercept, and auxiliary parameters.

##### Usage

```
stan_glm(formula, family = gaussian(), data, weights, subset,
na.action = NULL, offset = NULL, model = TRUE, x = FALSE,
y = TRUE, contrasts = NULL, ..., prior = normal(),
prior_intercept = normal(), prior_aux = exponential(),
prior_PD = FALSE, algorithm = c("sampling", "optimizing",
"meanfield", "fullrank"), mean_PPD = algorithm != "optimizing",
adapt_delta = NULL, QR = FALSE, sparse = FALSE)
```stan_glm.nb(formula, data, weights, subset, na.action = NULL,
offset = NULL, model = TRUE, x = FALSE, y = TRUE,
contrasts = NULL, link = "log", ..., prior = normal(),
prior_intercept = normal(), prior_aux = exponential(),
prior_PD = FALSE, algorithm = c("sampling", "optimizing",
"meanfield", "fullrank"), mean_PPD = algorithm != "optimizing",
adapt_delta = NULL, QR = FALSE)

stan_glm.fit(x, y, weights = rep(1, NROW(y)), offset = rep(0, NROW(y)),
family = gaussian(), ..., prior = normal(),
prior_intercept = normal(), prior_aux = exponential(),
prior_smooth = exponential(autoscale = FALSE), prior_ops = NULL,
group = list(), prior_PD = FALSE, algorithm = c("sampling",
"optimizing", "meanfield", "fullrank"), mean_PPD = algorithm !=
"optimizing", adapt_delta = NULL, QR = FALSE, sparse = FALSE)

##### Arguments

- formula, data, subset
Same as

`glm`

, but*we strongly advise against omitting the*. Unless`data`

argument`data`

is specified (and is a data frame) many post-estimation functions (including`update`

,`loo`

,`kfold`

) are not guaranteed to work properly.- family
Same as

`glm`

, except negative binomial GLMs are also possible using the`neg_binomial_2`

family object.- na.action, contrasts
Same as

`glm`

, but rarely specified.- model, offset, weights
Same as

`glm`

.- x
In

`stan_glm`

, logical scalar indicating whether to return the design matrix. In`stan_glm.fit`

, usually a design matrix but can also be a list of design matrices with the same number of rows, in which case the first element of the list is interpreted as the primary design matrix and the remaining list elements collectively constitute a basis for a smooth nonlinear function of the predictors indicated by the`formula`

argument to`stan_gamm4`

.- y
In

`stan_glm`

, logical scalar indicating whether to return the response vector. In`stan_glm.fit`

, a response vector.- ...
Further arguments passed to the function in the rstan package (

`sampling`

,`vb`

, or`optimizing`

), corresponding to the estimation method named by`algorithm`

. For example, if`algorithm`

is`"sampling"`

it is possibly to specify`iter`

,`chains`

,`cores`

,`refresh`

, etc.- prior
The prior distribution for the regression coefficients.

`prior`

should be a call to one of the various functions provided by rstanarm for specifying priors. The subset of these functions that can be used for the prior on the coefficients can be grouped into several "families":**Family****Functions***Student t family*`normal`

,`student_t`

,`cauchy`

*Hierarchical shrinkage family*`hs`

,`hs_plus`

*Laplace family*`laplace`

,`lasso`

*Product normal family*`product_normal`

See the priors help page for details on the families and how to specify the arguments for all of the functions in the table above. To omit a prior ---i.e., to use a flat (improper) uniform prior---

`prior`

can be set to`NULL`

, although this is rarely a good idea.**Note:**Unless`QR=TRUE`

, if`prior`

is from the Student t family or Laplace family, and if the`autoscale`

argument to the function used to specify the prior (e.g.`normal`

) is left at its default and recommended value of`TRUE`

, then the default or user-specified prior scale(s) may be adjusted internally based on the scales of the predictors. See the priors help page and the*Prior Distributions*vignette for details on the rescaling and the`prior_summary`

function for a summary of the priors used for a particular model.- prior_intercept
The prior distribution for the intercept.

`prior_intercept`

can be a call to`normal`

,`student_t`

or`cauchy`

. See the priors help page for details on these functions. To omit a prior on the intercept ---i.e., to use a flat (improper) uniform prior---`prior_intercept`

can be set to`NULL`

.**Note:**If using a dense representation of the design matrix ---i.e., if the`sparse`

argument is left at its default value of`FALSE`

--- then the prior distribution for the intercept is set so it applies to the value*when all predictors are centered*. If you prefer to specify a prior on the intercept without the predictors being auto-centered, then you have to omit the intercept from the`formula`

and include a column of ones as a predictor, in which case some element of`prior`

specifies the prior on it, rather than`prior_intercept`

. Regardless of how`prior_intercept`

is specified, the reported*estimates*of the intercept always correspond to a parameterization without centered predictors (i.e., same as in`glm`

).- prior_aux
The prior distribution for the "auxiliary" parameter (if applicable). The "auxiliary" parameter refers to a different parameter depending on the

`family`

. For Gaussian models`prior_aux`

controls`"sigma"`

, the error standard deviation. For negative binomial models`prior_aux`

controls`"reciprocal_dispersion"`

, which is similar to the`"size"`

parameter of`rnbinom`

: smaller values of`"reciprocal_dispersion"`

correspond to greater dispersion. For gamma models`prior_aux`

sets the prior on to the`"shape"`

parameter (see e.g.,`rgamma`

), and for inverse-Gaussian models it is the so-called`"lambda"`

parameter (which is essentially the reciprocal of a scale parameter). Binomial and Poisson models do not have auxiliary parameters.`prior_aux`

can be a call to`exponential`

to use an exponential distribution, or`normal`

,`student_t`

or`cauchy`

, which results in a half-normal, half-t, or half-Cauchy prior. See`priors`

for details on these functions. To omit a prior ---i.e., to use a flat (improper) uniform prior--- set`prior_aux`

to`NULL`

.- prior_PD
A logical scalar (defaulting to

`FALSE`

) indicating whether to draw from the prior predictive distribution instead of conditioning on the outcome.- algorithm
A string (possibly abbreviated) indicating the estimation approach to use. Can be

`"sampling"`

for MCMC (the default),`"optimizing"`

for optimization,`"meanfield"`

for variational inference with independent normal distributions, or`"fullrank"`

for variational inference with a multivariate normal distribution. See`rstanarm-package`

for more details on the estimation algorithms. NOTE: not all fitting functions support all four algorithms.- mean_PPD
A logical value indicating whether the sample mean of the posterior predictive distribution of the outcome should be calculated in the

`generated quantities`

block. If`TRUE`

then`mean_PPD`

is computed and displayed as a diagnostic in the printed output. The default is`TRUE`

except if`algorithm=="optimizing"`

. A useful heuristic is to check if`mean_PPD`

is plausible when compared to`mean(y)`

. If it is plausible then this does*not*mean that the model is good in general (only that it can reproduce the sample mean), but if`mean_PPD`

is implausible then there may be something wrong, e.g., severe model misspecification, problems with the data and/or priors, computational issues, etc.- adapt_delta
Only relevant if

`algorithm="sampling"`

. See the adapt_delta help page for details.- QR
A logical scalar defaulting to

`FALSE`

, but if`TRUE`

applies a scaled`qr`

decomposition to the design matrix. The transformation does not change the likelihood of the data but is recommended for computational reasons when there are multiple predictors. See the QR-argument documentation page for details on how rstanarm does the transformation and important information about how to interpret the prior distributions of the model parameters when using`QR=TRUE`

.- sparse
A logical scalar (defaulting to

`FALSE`

) indicating whether to use a sparse representation of the design (X) matrix. If`TRUE`

, the the design matrix is not centered (since that would destroy the sparsity) and likewise it is not possible to specify both`QR = TRUE`

and`sparse = TRUE`

. Depending on how many zeros there are in the design matrix, setting`sparse = TRUE`

may make the code run faster and can consume much less RAM.- link
For

`stan_glm.nb`

only, the link function to use. See`neg_binomial_2`

.- prior_smooth
The prior distribution for the hyperparameters in GAMs, with lower values yielding less flexible smooth functions.

`prior_smooth`

can be a call to`exponential`

to use an exponential distribution, or`normal`

,`student_t`

or`cauchy`

, which results in a half-normal, half-t, or half-Cauchy prior. See`priors`

for details on these functions. To omit a prior ---i.e., to use a flat (improper) uniform prior--- set`prior_smooth`

to`NULL`

. The number of hyperparameters depends on the model specification but a scalar prior will be recylced as necessary to the appropriate length.- prior_ops
Deprecated. See rstanarm-deprecated for details.

- group
A list, possibly of length zero (the default), but otherwise having the structure of that produced by

`mkReTrms`

to indicate the group-specific part of the model. In addition, this list must have elements for the`regularization`

,`concentration`

`shape`

, and`scale`

components of a`decov`

prior for the covariance matrices among the group-specific coefficients.

##### Details

The `stan_glm`

function is similar in syntax to
`glm`

but rather than performing maximum likelihood
estimation of generalized linear models, full Bayesian estimation is
performed (if `algorithm`

is `"sampling"`

) via MCMC. The Bayesian
model adds priors (independent by default) on the coefficients of the GLM.
The `stan_glm`

function calls the workhorse `stan_glm.fit`

function, but it is also possible to call the latter directly.

The `stan_glm.nb`

function, which takes the extra argument
`link`

, is a wrapper for `stan_glm`

with ```
family =
neg_binomial_2(link)
```

.

##### Value

A stanreg object is returned
for `stan_glm, stan_glm.nb`

.

A stanfit object (or a slightly modified
stanfit object) is returned if `stan_glm.fit`

is called directly.

##### References

Gelman, A. and Hill, J. (2007). *Data Analysis Using
Regression and Multilevel/Hierarchical Models.* Cambridge University Press,
Cambridge, UK. (Ch. 3-6)

Muth, C., Oravecz, Z., and Gabry, J. (2018)
User-friendly Bayesian regression modeling: A tutorial with rstanarm and shinystan.
*The Quantitative Methods for Psychology*. 14(2), 99--119.
https://www.tqmp.org/RegularArticles/vol14-2/p099/p099.pdf

##### See Also

`stanreg-methods`

and
`glm`

.

The various vignettes for `stan_glm`

at
http://mc-stan.org/rstanarm/articles/.

##### Examples

```
# NOT RUN {
if (!grepl("^sparc", R.version$platform)) {
### Linear regression
mtcars$mpg10 <- mtcars$mpg / 10
fit <- stan_glm(
mpg10 ~ wt + cyl + am,
data = mtcars,
QR = TRUE,
# for speed of example only (default is "sampling")
algorithm = "fullrank"
)
plot(fit, prob = 0.5)
plot(fit, prob = 0.5, pars = "beta")
plot(fit, "hist", pars = "sigma")
}
# }
# NOT RUN {
### Logistic regression
head(wells)
wells$dist100 <- wells$dist / 100
fit2 <- stan_glm(
switch ~ dist100 + arsenic,
data = wells,
family = binomial(link = "logit"),
prior_intercept = normal(0, 10),
QR = TRUE,
# for speed of example only
chains = 2, iter = 200
)
print(fit2)
prior_summary(fit2)
# ?bayesplot::mcmc_areas
plot(fit2, plotfun = "areas", prob = 0.9,
pars = c("(Intercept)", "arsenic"))
# ?bayesplot::ppc_error_binned
pp_check(fit2, plotfun = "error_binned")
### Poisson regression (example from help("glm"))
count_data <- data.frame(
counts = c(18,17,15,20,10,20,25,13,12),
outcome = gl(3,1,9),
treatment = gl(3,3)
)
fit3 <- stan_glm(
counts ~ outcome + treatment,
data = count_data,
family = poisson(link="log"),
prior = normal(0, 2, autoscale = FALSE),
# for speed of example only
chains = 2, iter = 250
)
print(fit3)
bayesplot::color_scheme_set("viridis")
plot(fit3)
plot(fit3, regex_pars = c("outcome", "treatment"))
plot(fit3, plotfun = "combo", regex_pars = "treatment") # ?bayesplot::mcmc_combo
posterior_vs_prior(fit3, regex_pars = c("outcome", "treatment"))
### Gamma regression (example from help("glm"))
clotting <- data.frame(log_u = log(c(5,10,15,20,30,40,60,80,100)),
lot1 = c(118,58,42,35,27,25,21,19,18),
lot2 = c(69,35,26,21,18,16,13,12,12))
fit4 <- stan_glm(
lot1 ~ log_u,
data = clotting,
family = Gamma(link="log"),
iter = 500 # for speed of example only
)
print(fit4, digits = 2)
fit5 <- update(fit4, formula = lot2 ~ log_u)
# ?bayesplot::ppc_dens_overlay
bayesplot::bayesplot_grid(
pp_check(fit4, seed = 123),
pp_check(fit5, seed = 123),
titles = c("lot1", "lot2")
)
### Negative binomial regression
fit6 <- stan_glm.nb(Days ~ Sex/(Age + Eth*Lrn), data = MASS::quine,
link = "log", prior_aux = exponential(1.5),
chains = 2, iter = 200) # for speed of example only
prior_summary(fit6)
bayesplot::color_scheme_set("brightblue")
plot(fit6)
pp_check(fit6, plotfun = "hist", nreps = 5) # ?bayesplot::ppc_hist
# 80% interval of estimated reciprocal_dispersion parameter
posterior_interval(fit6, pars = "reciprocal_dispersion", prob = 0.8)
plot(fit6, "areas", pars = "reciprocal_dispersion", prob = 0.8)
# }
# NOT RUN {
# }
```

*Documentation reproduced from package rstanarm, version 2.18.2, License: GPL (>= 3)*