---
title: "Leave-one-out cross-validation for non-factorized models"
author: "Aki Vehtari, Paul Bürkner and Jonah Gabry"
date: "`r Sys.Date()`"
output:
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    toc: yes
encoding: "UTF-8"
params:
  EVAL: !r identical(Sys.getenv("NOT_CRAN"), "true")
---
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# Introduction

When computing ELPD-based LOO-CV for a Bayesian model we need to
compute the log leave-one-out predictive densities $\log{p(y_i | y_{-i})}$ for
every response value $y_i, \: i = 1, \ldots, N$, where $y_{-i}$ denotes all
response values except observation $i$. To obtain $p(y_i | y_{-i})$, we need to
have access to the pointwise likelihood $p(y_i\,|\, y_{-i}, \theta)$ and
integrate over the model parameters $\theta$:

$$
p(y_i\,|\,y_{-i}) =
  \int p(y_i\,|\, y_{-i}, \theta) \, p(\theta\,|\, y_{-i}) \,d \theta
$$

Here, $p(\theta\,|\, y_{-i})$ is the leave-one-out posterior distribution for
$\theta$, that is, the posterior distribution for $\theta$ obtained by fitting
the model while holding out the $i$th observation (we will later show how 
refitting the model to data $y_{-i}$ can be avoided).

If the observation model is formulated directly as the product of the
pointwise observation models, we call it a *factorized*
model. In this case, the likelihood is also the product of the pointwise
likelihood contributions $p(y_i\,|\, y_{-i}, \theta)$.
To better illustrate possible structures of the observation models, we
formally divide $\theta$ into two parts, observation-specific latent
variables $f = (f_1, \ldots, f_N)$ and hyperparameters $\psi$, so that
$p(y_i\,|\, y_{-i}, \theta) = p(y_i\,|\, y_{-i}, f_i, \psi)$. 
Depending on the model, one of the two parts of $\theta$
may also be empty. In very simple models, such as linear regression models, 
latent variables are not explicitly presented and response values are 
conditionally independent given
$\psi$, so that $p(y_i\,|\, y_{-i}, f_i, \psi) = p(y_i \,|\, \psi)$. 
The full likelihood can then be written in the familiar form

$$
p(y \,|\, \psi) = \prod_{i=1}^N p(y_i \,|\, \psi),
$$

where $y = (y_1, \ldots, y_N)$ denotes the vector of all responses.
When the likelihood factorizes this way, the conditional
pointwise log-likelihood can be obtained easily by computing
$p(y_i\,|\, \psi)$ for each $i$ with computational cost $O(n)$.

Yet, there are several reasons why a *non-factorized* observation model
may be necessary or preferred. In non-factorized
models, the joint likelihood of the response values $p(y \,|\, \theta)$ is not
factorized into observation-specific components, but rather given directly as one
joint expression. For some models, an analytic factorized formulation is
simply not available in which case we speak of a *non-factorizable* model. Even
in models whose observation model can be factorized in principle, it may still be
preferable to use a non-factorized form for reasons of efficiency
and numerical stability (Bürkner et al. 2020).

Whether a non-factorized model is used by necessity or for efficiency and
stability, it comes at the cost of having no direct access to the leave-one-out
predictive densities and thus to the overall leave-one-out
predictive accuracy. In theory, we can express the observation-specific
likelihoods in terms of the joint likelihood via

$$
p(y_i \,|\, y_{i-1}, \theta) = 
  \frac{p(y \,|\, \theta)}{p(y_{-i} \,|\, \theta)} = 
  \frac{p(y \,|\, \theta)}{\int p(y \,|\, \theta) \, d y_i},
$$

but the expression on the right-hand side may not always
have an analytical solution. Computing $\log p(y_i \,|\, y_{-i},
\theta)$ for non-factorized models is therefore often impossible, or at least
inefficient and numerically unstable. However, there is a large
class of multivariate normal and Student-$t$ models for which there are
efficient analytical solutions available.

More details can be found in our paper about LOO-CV for non-factorized models
(Bürkner, Gabry, & Vehtari, 2020), which is available as a preprint on arXiv
(https://arxiv.org/abs/1810.10559).

# LOO-CV for multivariate normal models

In this vignette, we will focus on non-factorized multivariate normal
models. Based on results of Sundararajan and Keerthi (2001), Bürkner et al. (2020) 
show that, for multivariate normal models with coriance matrix $C$, 
the LOO predictive mean and standard deviation can be computed as follows:

\begin{align}
  \mu_{\tilde{y},-i} &= y_i-\bar{c}_{ii}^{-1} g_i \nonumber \\
  \sigma_{\tilde{y},-i} &= \sqrt{\bar{c}_{ii}^{-1}},
\end{align}
where $g_i$ and $\bar{c}_{ii}$ are
\begin{align}
  g_i &= \left[C^{-1} y\right]_i \nonumber \\
  \bar{c}_{ii} &= \left[C^{-1}\right]_{ii}.
\end{align}

Using these results, the log predictive density of the $i$th observation is then
computed as

$$
  \log p(y_i \,|\, y_{-i},\theta)
  = - \frac{1}{2}\log(2\pi)
  - \frac{1}{2}\log \sigma^2_{-i}
  - \frac{1}{2}\frac{(y_i-\mu_{-i})^2}{\sigma^2_{-i}}.
$$

Expressing this same equation in terms of $g_i$ and $\bar{c}_{ii}$, the log
predictive density becomes:

$$
  \log p(y_i \,|\, y_{-i},\theta)
  = - \frac{1}{2}\log(2\pi)
  + \frac{1}{2}\log \bar{c}_{ii}
  - \frac{1}{2}\frac{g_i^2}{\bar{c}_{ii}}.
$$
(Note that Vehtari et al. (2016) has a typo in the corresponding Equation 34.)

From these equations we can now derive a recipe for obtaining the conditional
pointwise log-likelihood for _all_ models that can be expressed conditionally in
terms of a multivariate normal with invertible covariance matrix $C$.

## Approximate LOO-CV using integrated importance-sampling

The above LOO equations for multivariate normal models are conditional on
parameters $\theta$. Therefore, to obtain the leave-one-out predictive density
$p(y_i \,|\, y_{-i})$ we need to integrate over $\theta$,

$$
p(y_i\,|\,y_{-i}) =
  \int p(y_i\,|\,y_{-i}, \theta) \, p(\theta\,|\,y_{-i}) \,d\theta.
$$

Here, $p(\theta\,|\,y_{-i})$ is the leave-one-out posterior distribution for
$\theta$, that is, the posterior distribution for $\theta$ obtained by fitting
the model while holding out the $i$th observation. 

To avoid the cost of sampling from $N$ leave-one-out posteriors, it is possible
to take the posterior draws $\theta^{(s)}, \, s=1,\ldots,S$, from the
\emph{full} posterior $p(\theta\,|\,y)$, and then approximate the above integral
using integrated importance sampling (Vehtari et al., 2016, Section 3.6.1):

$$
 p(y_i\,|\,y_{-i}) \approx
   \frac{ \sum_{s=1}^S p(y_i\,|\,y_{-i},\,\theta^{(s)}) \,w_i^{(s)}}{ \sum_{s=1}^S w_i^{(s)}},
$$

where $w_i^{(s)}$ are importance weights. First we compute the raw importance ratios

$$
  r_i^{(s)} \propto \frac{1}{p(y_i \,|\, y_{-i}, \,\theta^{(s)})},
$$

and then stabilize them using Pareto smoothed importance sampling (PSIS, Vehtari
et al, 2019) to obtain the weights $w_i^{(s)}$. The resulting approximation is
referred to as PSIS-LOO (Vehtari et al, 2017).

## Exact LOO-CV with re-fitting

In order to validate the approximate LOO procedure, and also in order to allow
exact computations to be made for a small number of leave-one-out folds for
which the Pareto $k$ diagnostic (Vehtari et al, 2022) indicates an unstable
approximation, we need to consider how we might to do _exact_ leave-one-out CV
for a non-factorized model. In the case of a Gaussian process that has the
marginalization property, we could just drop the one row and column of $C$
corresponding to the held out out observation. This does not hold in general for
multivariate normal models, however, and to keep the original prior we may need
to maintain the full covariance matrix $C$ even when one of the observations is
left out.

The solution is to model $y_i$ as a missing observation and estimate it along
with all of the other model parameters. For a conditional multivariate normal
model, $\log p(y_i\,|\,y_{-i})$ can be computed as follows. First, we model
$y_i$ as missing and denote the corresponding parameter $y_i^{\mathrm{mis}}$.
Then, we define

$$
y_{\mathrm{mis}(i)} = (y_1, \ldots, y_{i-1}, y_i^{\mathrm{mis}}, y_{i+1}, \ldots, y_N).
$$
to be the same as the full set of observations $y$, except replacing $y_i$ with
the parameter $y_i^{\mathrm{mis}}$.

Second, we compute the LOO predictive mean and standard deviations as above, but
replace $y$ with $y_{\mathrm{mis}(i)}$ in the computation of
$\mu_{\tilde{y},-i}$:

$$
\mu_{\tilde{y},-i} = y_{{\mathrm{mis}}(i)}-\bar{c}_{ii}^{-1}g_i,
$$

where in this case we have

$$
g_i = \left[ C^{-1} y_{\mathrm{mis}(i)} \right]_i.
$$

The conditional log predictive density is then computed with the above
$\mu_{\tilde{y},-i}$ and the left out observation $y_i$:

$$
  \log p(y_i\,|\,y_{-i},\theta)
  = - \frac{1}{2}\log(2\pi)
  - \frac{1}{2}\log \sigma^2_{\tilde{y},-i}
  - \frac{1}{2}\frac{(y_i-\mu_{\tilde{y},-i})^2}{\sigma^2_{\tilde{y},-i}}.
$$

Finally, the leave-one-out predictive distribution can then be estimated as

$$
 p(y_i\,|\,y_{-i}) \approx \sum_{s=1}^S p(y_i\,|\,y_{-i}, \theta_{-i}^{(s)}),
$$

where $\theta_{-i}^{(s)}$ are draws from the posterior distribution
$p(\theta\,|\,y_{\mathrm{mis}(i)})$.

# Lagged SAR models

A common non-factorized multivariate normal model is the simultaneously
autoregressive (SAR) model, which is frequently used for spatially correlated
data. The lagged SAR model is defined as

$$
y = \rho Wy + \eta + \epsilon
$$
or equivalently
$$
(I - \rho W)y = \eta + \epsilon,
$$
where $\rho$ is the spatial correlation parameter and $W$ is a user-defined
weight matrix. The matrix $W$ has entries $w_{ii} = 0$ along the diagonal and
the off-diagonal entries $w_{ij}$ are larger when areas $i$ and $j$ are closer
to each other. In a linear model, the predictor term $\eta$ is given by 
$\eta = X \beta$ with design matrix $X$ and regression coefficients $\beta$. 
However, since the above equation holds for arbitrary $\eta$, these results are 
not restricted to linear models.

If we have $\epsilon \sim {\mathrm N}(0, \,\sigma^2 I)$, it follows that
$$
(I - \rho W)y \sim {\mathrm N}(\eta, \sigma^2 I),
$$
which corresponds to the following log PDF coded in **Stan**:

```{r lpdf, eval=FALSE}
/** 
 * Normal log-pdf for spatially lagged responses
 * 
 * @param y Vector of response values.
 * @param mu Mean parameter vector.
 * @param sigma Positive scalar residual standard deviation.
 * @param rho Positive scalar autoregressive parameter.
 * @param W Spatial weight matrix.
 *
 * @return A scalar to be added to the log posterior.
 */
real normal_lagsar_lpdf(vector y, vector mu, real sigma, 
                        real rho, matrix W) {
  int N = rows(y);
  real inv_sigma2 = 1 / square(sigma);
  matrix[N, N] W_tilde = -rho * W;
  vector[N] half_pred;
  
  for (n in 1:N) W_tilde[n,n] += 1;
  
  half_pred = W_tilde * (y - mdivide_left(W_tilde, mu));
  
  return 0.5 * log_determinant(crossprod(W_tilde) * inv_sigma2) -
         0.5 * dot_self(half_pred) * inv_sigma2;
}
```

For the purpose of computing LOO-CV, it makes sense to rewrite the SAR model in
slightly different form. Conditional on $\rho$, $\eta$, and $\sigma$, 
if we write

\begin{align}
y-(I-\rho W)^{-1}\eta &\sim {\mathrm N}(0, \sigma^2(I-\rho W)^{-1}(I-\rho W)^{-T}),
\end{align}
or more compactly, with $\widetilde{W}=(I-\rho W)$,
\begin{align}
y-\widetilde{W}^{-1}\eta &\sim {\mathrm N}(0, \sigma^2(\widetilde{W}^{T}\widetilde{W})^{-1}),
\end{align}

then this has the same form as the zero mean Gaussian process from above.
Accordingly, we can compute the leave-one-out predictive densities with the
equations from Sundararajan and Keerthi (2001), replacing $y$ with
$(y-\widetilde{W}^{-1}\eta)$ and taking the covariance matrix $C$ to be
$\sigma^2(\widetilde{W}^{T}\widetilde{W})^{-1}$.


## Case Study: Neighborhood Crime in Columbus, Ohio

In order to demonstrate how to carry out the computations implied by these
equations, we will first fit a lagged SAR model to data on crime in 49 different
neighborhoods of Columbus, Ohio during the year 1980. The data was originally
described in Aneslin (1988) and ships with the **spdep** R package.

In addition to the **loo** package, for this analysis we will use the **brms**
interface to Stan to generate a Stan program and fit the model, and also the
**bayesplot** and **ggplot2** packages for plotting.

```{r setup, cache=FALSE}
library("loo")
library("brms")
library("bayesplot")
library("ggplot2")
color_scheme_set("brightblue")
theme_set(theme_default())


SEED <- 10001 
set.seed(SEED) # only sets seed for R (seed for Stan set later)

# loads COL.OLD data frame and COL.nb neighbor list
data(oldcol, package = "spdep") 
```

The three variables in the data set relevant to this example are:

* `CRIME`: the number of residential burglaries and vehicle thefts per thousand
households in the neighbood
* `HOVAL`: housing value in units of $1000 USD 
* `INC`: household income in units of $1000 USD

```{r data}
str(COL.OLD[, c("CRIME", "HOVAL", "INC")])
```

We will also use the object `COL.nb`, which is a list containing information
about which neighborhoods border each other. From this list we will be able to
construct the weight matrix to used to help account for the spatial dependency
among the observations.

### Fit lagged SAR model

A model predicting `CRIME` from `INC` and `HOVAL`, while accounting for the
spatial dependency via an SAR structure, can be specified in **brms** as
follows.

```{r fit, results="hide"}
fit <- brm(
  CRIME ~ INC + HOVAL + sar(COL.nb, type = "lag"), 
  data = COL.OLD,
  data2 = list(COL.nb = COL.nb),
  chains = 4,
  seed = SEED
)
```

The code above fits the model in **Stan** using a log PDF equivalent to the
`normal_lagsar_lpdf` function we defined above. In the summary output below we
see that both higher income and higher housing value predict lower crime rates
in the neighborhood. Moreover, there seems to be substantial spatial correlation
between adjacent neighborhoods, as indicated by the posterior distribution 
of the `lagsar` parameter.

```{r plot-lagsar, message=FALSE}
lagsar <- as.matrix(fit, pars = "lagsar")
estimates <- quantile(lagsar, probs = c(0.25, 0.5, 0.75))
mcmc_hist(lagsar) + 
  vline_at(estimates, linetype = 2, size = 1) +
  ggtitle("lagsar: posterior median and 50% central interval")
```

### Approximate LOO-CV

After fitting the model, the next step is to compute the pointwise
log-likelihood values needed for approximate LOO-CV. To do this we will 
use the recipe laid out in the previous sections.

```{r approx}
posterior <- as.data.frame(fit)
y <- fit$data$CRIME
N <- length(y)
S <- nrow(posterior)
loglik <- yloo <- sdloo <- matrix(nrow = S, ncol = N)

for (s in 1:S) {
  p <- posterior[s, ]
  eta <- p$b_Intercept + p$b_INC * fit$data$INC + p$b_HOVAL * fit$data$HOVAL
  W_tilde <- diag(N) - p$lagsar * spdep::nb2mat(COL.nb)
  Cinv <- t(W_tilde) %*% W_tilde / p$sigma^2
  g <- Cinv %*% (y - solve(W_tilde, eta))
  cbar <- diag(Cinv)
  yloo[s, ] <- y - g / cbar
  sdloo[s, ] <- sqrt(1 / cbar)
  loglik[s, ] <- dnorm(y, yloo[s, ], sdloo[s, ], log = TRUE)
}

# use loo for psis smoothing
log_ratios <- -loglik
psis_result <- psis(log_ratios)
```

The quality of the PSIS-LOO approximation can be investigated graphically by
plotting the Pareto-k estimate for each observation. The approximation is robust up to values
of $0.7$ (Vehtari et al, 2017, 2022). In the plot below, we see that the fourth
observation is problematic and so may reduce the accuracy of the LOO-CV
approximation.

```{r plot, cache = FALSE}
plot(psis_result, label_points = TRUE)
```

We can also check that the conditional leave-one-out predictive distribution
equations work correctly, for instance, using the last posterior draw:

```{r checklast, cache = FALSE}
yloo_sub <- yloo[S, ]
sdloo_sub <- sdloo[S, ]
df <- data.frame(
  y = y, 
  yloo = yloo_sub,
  ymin = yloo_sub - sdloo_sub * 2,
  ymax = yloo_sub + sdloo_sub * 2
)
ggplot(data=df, aes(x = y, y = yloo, ymin = ymin, ymax = ymax)) +
  geom_errorbar(
    width = 1, 
    color = "skyblue3", 
    position = position_jitter(width = 0.25)
  ) +
  geom_abline(color = "gray30", size = 1.2) +
  geom_point()
```

Finally, we use PSIS-LOO to approximate the expected log predictive density
(ELPD) for new data, which we will validate using exact LOO-CV in the upcoming
section.

```{r psisloo}
(psis_loo <- loo(loglik))
```

### Exact LOO-CV

Exact LOO-CV for the above example is somewhat more involved, as we need to
re-fit the model $N$ times and each time model the held-out data point as a
parameter. First, we create an empty dummy model that we will update below as we
loop over the observations.

```{r fit_dummy, cache = TRUE}
# see help("mi", "brms") for details on the mi() usage
fit_dummy <- brm(
  CRIME | mi() ~ INC + HOVAL + sar(COL.nb, type = "lag"), 
  data = COL.OLD,
  data2 = list(COL.nb = COL.nb),
  chains = 0
)
```

Next, we fit the model $N$ times, each time leaving out a single observation and
then computing the log predictive density for that observation. For obvious
reasons, this takes much longer than the approximation we computed above, but it
is necessary in order to validate the approximate LOO-CV method. Thanks 
to the PSIS-LOO approximation, in general doing these slow exact computations
can be avoided.

```{r exact-loo-cv, results="hide", message=FALSE, warning=FALSE, cache = TRUE}
S <- 500
res <- vector("list", N)
loglik <- matrix(nrow = S, ncol = N)
for (i in seq_len(N)) {
  dat_mi <- COL.OLD
  dat_mi$CRIME[i] <- NA
  fit_i <- update(fit_dummy, newdata = dat_mi, 
                  # just for vignette
                  chains = 1, iter = S * 2)
  posterior <- as.data.frame(fit_i)
  yloo <- sdloo <- rep(NA, S)
  for (s in seq_len(S)) {
    p <- posterior[s, ]
    y_miss_i <- y
    y_miss_i[i] <- p$Ymi
    eta <- p$b_Intercept + p$b_INC * fit_i$data$INC + p$b_HOVAL * fit_i$data$HOVAL
    W_tilde <- diag(N) - p$lagsar * spdep::nb2mat(COL.nb)
    Cinv <- t(W_tilde) %*% W_tilde / p$sigma^2
    g <- Cinv %*% (y_miss_i - solve(W_tilde, eta))
    cbar <- diag(Cinv);
    yloo[s] <- y_miss_i[i] - g[i] / cbar[i]
    sdloo[s] <- sqrt(1 / cbar[i])
    loglik[s, i] <- dnorm(y[i], yloo[s], sdloo[s], log = TRUE)
  }
  ypred <- rnorm(S, yloo, sdloo)
  res[[i]] <- data.frame(y = c(posterior$Ymi, ypred))
  res[[i]]$type <- rep(c("pp", "loo"), each = S)
  res[[i]]$obs <- i
}
res <- do.call(rbind, res)
```

A first step in the validation of the pointwise predictive density is to compare
the distribution of the implied response values for the left-out observation to
the distribution of the $y_i^{\mathrm{mis}}$ posterior-predictive values
estimated as part of the model. If the pointwise predictive density is correct,
the two distributions should match very closely (up to sampling error). In the
plot below, we overlay these two distributions for the first four observations
and see that they match very closely (as is the case for all $49$ observations
of in this example).

```{r yplots, cache = FALSE, fig.width=10, out.width="95%", fig.asp = 0.3}
res_sub <- res[res$obs %in% 1:4, ]
ggplot(res_sub, aes(y, fill = type)) +
  geom_density(alpha = 0.6) +
  facet_wrap("obs", scales = "fixed", ncol = 4)
```

In the final step, we compute the ELPD based on the exact LOO-CV and compare it
to the approximate PSIS-LOO result computed earlier.

```{r loo_exact, cache=FALSE}
log_mean_exp <- function(x) {
  # more stable than log(mean(exp(x)))
  max_x <- max(x)
  max_x + log(sum(exp(x - max_x))) - log(length(x))
}
exact_elpds <- apply(loglik, 2, log_mean_exp)
exact_elpd <- sum(exact_elpds)
round(exact_elpd, 1)
```

The results of the approximate and exact LOO-CV are similar but not as close as
we would expect if there were no problematic observations. We can investigate
this issue more closely by plotting the approximate against the exact pointwise
ELPD values.

```{r compare, fig.height=5}
df <- data.frame(
  approx_elpd = psis_loo$pointwise[, "elpd_loo"],
  exact_elpd = exact_elpds
)
ggplot(df, aes(x = approx_elpd, y = exact_elpd)) +
  geom_abline(color = "gray30") +
  geom_point(size = 2) +
  geom_point(data = df[4, ], size = 3, color = "red3") +
  xlab("Approximate elpds") +
  ylab("Exact elpds") +
  coord_fixed(xlim = c(-16, -3), ylim = c(-16, -3))
```

In the plot above the fourth data point ---the observation flagged as
problematic by the PSIS-LOO approximation--- is colored in red and is the clear
outlier. Otherwise, the correspondence between the exact and approximate values
is strong. In fact, summing over the pointwise ELPD values and leaving out the
fourth observation yields practically equivalent results for approximate and
exact LOO-CV:

```{r pt4}
without_pt_4 <- c(
  approx = sum(psis_loo$pointwise[-4, "elpd_loo"]),
  exact = sum(exact_elpds[-4])  
)
round(without_pt_4, 1)
```

From this we can conclude that the difference we found when including *all*
observations does not indicate a bug in our implementation of the approximate
LOO-CV but rather a violation of its assumptions.


# Working with Stan directly

So far, we have specified the models in brms and only used Stan implicitely
behind the scenes. This allowed us to focus on the primary purpose of validating
approximate LOO-CV for non-factorized models. However, we would also like to
show how everything can be set up in Stan directly. The Stan code brms generates
is human readable and so we can use it to learn some of the essential aspects of
Stan and the particular model we are implementing. The Stan program below is
a slightly modified version of the code extracted via `stancode(fit_dummy)`:

```{r brms-stan-code, eval=FALSE}
// generated with brms 2.2.0
functions {
/** 
 * Normal log-pdf for spatially lagged responses
 * 
 * @param y Vector of response values.
 * @param mu Mean parameter vector.
 * @param sigma Positive scalar residual standard deviation.
 * @param rho Positive scalar autoregressive parameter.
 * @param W Spatial weight matrix.
 *
 * @return A scalar to be added to the log posterior.
 */
  real normal_lagsar_lpdf(vector y, vector mu, real sigma,
                          real rho, matrix W) {
    int N = rows(y);
    real inv_sigma2 = 1 / square(sigma);
    matrix[N, N] W_tilde = -rho * W;
    vector[N] half_pred;
    for (n in 1:N) W_tilde[n, n] += 1;
    half_pred = W_tilde * (y - mdivide_left(W_tilde, mu));
    return 0.5 * log_determinant(crossprod(W_tilde) * inv_sigma2) -
           0.5 * dot_self(half_pred) * inv_sigma2;
  }
}
data {
  int<lower=1> N;  // total number of observations
  vector[N] Y;  // response variable
  int<lower=0> Nmi;  // number of missings
  int<lower=1> Jmi[Nmi];  // positions of missings
  int<lower=1> K;  // number of population-level effects
  matrix[N, K] X;  // population-level design matrix
  matrix[N, N] W;  // spatial weight matrix
  int prior_only;  // should the likelihood be ignored?
}
transformed data {
  int Kc = K - 1;
  matrix[N, K - 1] Xc;  // centered version of X
  vector[K - 1] means_X;  // column means of X before centering
  for (i in 2:K) {
    means_X[i - 1] = mean(X[, i]);
    Xc[, i - 1] = X[, i] - means_X[i - 1];
  }
}
parameters {
  vector[Nmi] Ymi;  // estimated missings
  vector[Kc] b;  // population-level effects
  real temp_Intercept;  // temporary intercept
  real<lower=0> sigma;  // residual SD
  real<lower=0,upper=1> lagsar;  // SAR parameter
}
transformed parameters {
}
model {
  vector[N] Yl = Y;
  vector[N] mu = Xc * b + temp_Intercept;
  Yl[Jmi] = Ymi;
  // priors including all constants
  target += student_t_lpdf(temp_Intercept | 3, 34, 17);
  target += student_t_lpdf(sigma | 3, 0, 17)
    - 1 * student_t_lccdf(0 | 3, 0, 17);
  // likelihood including all constants
  if (!prior_only) {
    target += normal_lagsar_lpdf(Yl | mu, sigma, lagsar, W);
  }
}
generated quantities {
  // actual population-level intercept
  real b_Intercept = temp_Intercept - dot_product(means_X, b);
}
```

Here we want to focus on two aspects of the Stan code. First, because there is
no built-in function in Stan that calculates the log-likelihood for the lag-SAR
model, we define a new `normal_lagsar_lpdf` function in the `functions` block of
the Stan program. This is the same function we showed earlier in the vignette
and it can be used to compute the log-likelihood in an efficient and numerically
stable way. The `_lpdf` suffix used in the function name informs Stan that this
is a log probability density function.

Second, this Stan program nicely illustrates how to set up missing value
imputation. Instead of just computing the log-likelihood for the observed
responses `Y`, we define a new variable `Yl` which is equal to `Y` if the
reponse is observed and equal to `Ymi` if the response is missing. The latter is
in turn defined as a parameter and thus estimated along with all other paramters
of the model. More details about missing value imputation in Stan can be found
in the *Missing Data & Partially Known Parameters* section of the
[Stan manual](https://mc-stan.org/users/documentation/index.html).

The Stan code extracted from brms is not only helpful when learning Stan, but
can also drastically speed up the specification of models that are not support
by brms. If brms can fit a model similar but not identical to the desired model,
we can let brms generate the Stan program for the similar model and then mold it
into the program that implements the model we actually want to fit. Rather than
calling `stancode()`, which requires an existing fitted model object, we
recommend using `make_stancode()` and specifying the `save_model` argument to
write the Stan program to a file. The corresponding data can be prepared with
`make_standata()` and then manually amended if needed. Once the code and data
have been edited, they can be passed to RStan's `stan()` function via the `file`
and `data` arguments.

# Conclusion

In summary, we have shown how to set up and validate approximate and exact
LOO-CV for non-factorized multivariate normal models using Stan with the
**brms** and **loo** packages. Although we focused on the particular example of
a spatial SAR model, the presented recipe applies more generally to models that
can be expressed in terms of a multivariate normal likelihood.

<br />

# References

Anselin L. (1988). *Spatial econometrics: methods and models*. Dordrecht: Kluwer Academic.

Bürkner P. C., Gabry J., & Vehtari A. (2020). Efficient leave-one-out cross-validation for Bayesian non-factorized normal and Student-t models. *Computational Statistics*, \doi:10.1007/s00180-020-01045-4. 
[ArXiv preprint](https://arxiv.org/abs/1810.10559).

Sundararajan S. & Keerthi S. S. (2001). Predictive approaches for choosing hyperparameters in Gaussian processes. *Neural Computation*, 13(5), 1103--1118.

Vehtari A., Mononen T., Tolvanen V., Sivula T., & Winther O. (2016). Bayesian leave-one-out cross-validation approximations for Gaussian latent variable models. *Journal of Machine Learning Research*, 17(103), 1--38. [Online](https://jmlr.org/papers/v17/14-540.html).

Vehtari A., Gelman A., & Gabry J. (2017). Practical Bayesian model evaluation using leave-one-out cross-validation and WAIC. *Statistics and Computing*, 27(5), 1413--1432. \doi:10.1007/s11222-016-9696-4. [Online](https://link.springer.com/article/10.1007/s11222-016-9696-4). [arXiv preprint arXiv:1507.04544](https://arxiv.org/abs/1507.04544).

Vehtari, A., Simpson, D., Gelman, A., Yao, Y., and Gabry, J. (2022). Pareto smoothed importance sampling. [arXiv preprint arXiv:1507.02646](https://arxiv.org/abs/1507.02646).
