In this vignette, the functions exact_horseshoe, approx_horseshoe are explained in the following sections.

- About the horseshoe estimator in this package
- Brief Description of approximate MCMC algorithm

The horseshoe prior is a continuous reduced prior distribution frequently used in high-dimensional Bayesian linear models, and can be effectively applied to sparse data. This is a method that theoretically guarantees excellent shrinkage properties. Mhorseshoe provides an estimator applying the horseshoe prior, The likelihood function of the Gaussian linear model to be considered in this package is as follows:

\[L(y\ |\ x, \beta, \sigma^2) = (\frac{1}{\sqrt{2\pi}\sigma})^{-N/2}exp \{ -\frac{1}{2\sigma^2}(y-X\beta)^T(y-X\beta)\},\\ X \in \mathbb{R}^{N \times p},\ y \in \mathbb{R}^{N},\ \beta \in \mathbb{R}^{p}\]

And the form of the hierarchical model applying the horseshoe prior to \(\beta\) is expressed as follows:

\[\beta_{j}\ |\ \sigma^{2}, \tau^{2}, \lambda_{j}^{2} \overset{i.i.d}{\sim} N\left(0, \sigma^{2}\tau^{2}\lambda_{j}^{2} \right), \quad \lambda_{j}\overset{i.i.d}{\sim}C^{+}(0,1),\quad j=1,2,...,p, \\ \tau \overset{i.i.d}{\sim}C^{+}(0,1),\quad p(\sigma^2)\propto gamma\left( w/2, w/2\right).\]

Where \(C^{+}(0,1)\) is a half-Cauchy distribution, \(\lambda_{1}, ..., \lambda_{p}\) are local shrinkage parameters, and \(\tau\) is global shrinkage parameter. The exact_horseshoe in this package is a horseshoe estimator for the prior defined above, as described in exact MCMC algorithm of Johndrow et al. (2020). The blocked Metropolis-within-Gibb that samples \(\eta,\ \left(\tau, \sigma, \beta \right)\) is adopted, and the brief process is as follows.

\[\xi = \tau^{-2},\quad \eta_{j} = \lambda_{j}^{-2},\quad j = 1,2,...,p, \\ D = diag\left(\eta_{1}^{-1},..., \eta_{p}^{-1} \right), \quad M_{\xi} = I_{N} + \xi^{-1}XDX^{T}, \\ p(\xi | y, X, \eta) \propto |M_{\xi}|^{-1/2}\{(w + y^{T}M_{\xi}^{-1}y) / 2 \}^{-(w + N) / 2}/ \{\sqrt{\xi}(1+\xi)\}.\]

- Sample , using the following posterior of for rejection sampling. \[p(\eta_{j} | \xi, \beta_{j}, \sigma^{2}) \propto \frac{1}{1 + \eta_{j}}exp\{-\frac{\beta_{j}^{2}\xi \eta_{j}}{2\sigma^{2}} \}.\]
- Sample using the proposed MH algorithm as follows. \[log(\xi^{\star}) \sim N(log(\xi), s), accept\ \xi \ w.p.\ \{p(\xi^{\star} | y, X, \eta)\xi^{\star}\}/\{p(\xi | y, X, \eta)\xi \}.\]
- Sample \(\sigma^{2}\), \[\sigma^{2} | y, X, \eta, \xi \sim InvGamma\{(w + N) / 2, (w + y^{T}M_{\xi}^{-1}y) / 2 \}.\]
- Sample \(\beta\), \[\beta | y, X, \eta, \xi, \sigma \sim N\left(\left(X^{T}X + \left(\xi^{-1} D \right)^{-1}\right)^{-1}X^{T}y, \ \sigma^{2}\left(X^{T}X + \left(\xi^{-1} D \right)^{-1} \right)^{-1} \right).\]

Since the algorithm of this package considers the case of \(p >> N\), the computational cost can be lowered by sampling \(\beta\) using fast sampling(Bhattacharya et al.,2016) in the step 4.

\[u \sim N_{p}(0, \xi^{-1}D),\quad f \sim N_{N}(0, I_{N}), \\ v = Xu + f,\quad v^{\star} = M_{\xi}^{-1}(y/\sigma - v), \\ \beta = \sigma(u + \xi^{-1}DX^{T}v^{\star}).\]

For simulation, the data size was set to N = 300, p = 500, and the design matrix \(X=\{x_{j} \}_{j=1}^{p}\), true value of beta were set as follows.

\[x_{j} \sim N_{N}\left(0, I_{N} \right), \\ y_{i} \sim N(x_{i}\beta, 4), \\ \beta_{j} = \begin{cases} 1, & \mbox{if }\ \mbox{j<51,} \\ 0, & \mbox{otherwise.}\end{cases}\]

As a caution, the data is for testing the application of the algorithm and is not a simulation that strictly considers sparsity condition.

```
# making simulation data.
set.seed(123)
N <- 300
p <- 500
p_star <- 50
true_beta <- c(rep(1, p_star), rep(0, p-p_star))
# design matrix X.
X <- matrix(1, nrow = N, ncol = p)
for (i in 1:p) {
X[, i] <- stats::rnorm(N, mean = 0, sd = 1)
}
# response variable y.
y <- vector(mode = "numeric", length = N)
e <- rnorm(N, mean = 0, sd = 2)
for (i in 1:p_star) {
y <- y + true_beta[i] * X[, i]
}
y <- y + e
```

For the corresponding simulation data, the results were compared with the horseshoe function of the horseshoe package. We fit the data to the horseshoe and exact_horseshoe functions and plot the results for the first 100 indices, including the first 50 indices that are non-zero coefficients.

```
# horseshoe in horseshoe package.
horseshoe_result <- horseshoe::horseshoe(y, X, method.tau = "halfCauchy",
method.sigma = "Jeffreys",
burn = 0, nmc = 500)
# exact_horseshoe.
exact_horseshoe_result <- exact_horseshoe(X, y, burn = 0, iter = 500)
df <- data.frame(index = 1:100,
horseshoe_BetaHat = horseshoe_result$BetaHat[1:100],
horseshoe_LeftCI = horseshoe_result$LeftCI[1:100],
horseshoe_RightCI = horseshoe_result$RightCI[1:100],
exhorseshoe_BetaHat = exact_horseshoe_result$BetaHat[1:100],
exhorseshoe_LeftCI = exact_horseshoe_result$LeftCI[1:100],
exhorseshoe_RightCI = exact_horseshoe_result$RightCI[1:100],
true_beta = true_beta[1:100])
# Estimation results of the horseshoe.
ggplot(data = df, aes(x = index, y = true_beta)) +
geom_point(size = 2) +
geom_point(aes(x = index, y = horseshoe_BetaHat), size = 2, col = "red") +
geom_errorbar(aes(ymin = horseshoe_LeftCI,
ymax = horseshoe_RightCI), width = .1, col = "red") +
labs(title = "95% Credible intervals of the horseshoe function", y = "beta")
```

```
# Estimation results of the exact_horseshoe.
ggplot(data = df, aes(x = index, y = true_beta)) +
geom_point(size = 2) +
geom_point(aes(x = index, y = exhorseshoe_BetaHat),
size = 2, col = "red") +
geom_errorbar(aes(ymin = exhorseshoe_LeftCI,
ymax = exhorseshoe_RightCI), width = .1, col = "red") +
labs(title = "95% Credible intervals of the exact_horseshoe function",
y = "beta")
```

The results for both functions were similar. Both functions estimate that 95% of the credible intervals for all non-zero indices except 4, 36, 41, and 46 include 1.

In general, the disadvantage of the horseshoe estimator is that the computational cost is large for high-dimensional data where \(p>>N\). To overcome these limitations, Johndrow et al. (2020) proposed a scalable approximate MCMC algorithm that can reduce computational costs by introducing a thresholding method while applying the horseshoe prior. This algorithm has the following simple changes compared to the exact algorithm in section 1.

\[D_{\delta} = diag\left(\eta_{j}^{-1}1\left(\xi^{-1}\eta_{j}^{-1} > \delta,\ j=1,2,...,p. \right) \right),\] \[M_{\xi} \approx M_{\xi, \delta} = I_{N} + \xi^{-1}XD_{\delta}X^{T}.\]

The set of columns that satisfies the condition (\(\xi^{-1}\eta_{j}^{-1} > \delta\)) is defined as the active set, and let’s define \(S\) as the index set of the following columns.

\[S = \{j\ |\ \xi^{-1}\eta_{j}^{-1} > \delta,\ j=1,2,...,p. \}.\]

If \(\xi^{-1}\eta_{j}^{-1}\) is very small, the posterior of \(\beta\) will have a mean and variance close to 0. Therefore, let’s set \(\xi^{-1}\eta_{j}^{-1}\) smaller than \(\delta\) to 0 and the size of inverse \(M_{\xi, \delta}\) matrix is reduced as follows.

\[length(S)=s_{\delta} \le p, \\ X_{S} \in R^{N \times s_{\delta}}, \quad D_{S} \in R^{s_{\delta} \times s_{\delta}}, \\ M_{\xi, \delta}^{-1} = \left(I_{N} + \xi^{-1}X_{S}D_{S}X_{S}^{T} \right)^{-1}.\]

\(M_{\xi, \delta}^{-1}\) can be expressed using the Woodbury identity as follows.

\[M_{\xi, \delta}^{-1} = I_{N} - X_{S}\left(\xi D_{S}^{-1} + X_{S}^{T}X_{S} \right)^{-1}X_{S}^{T}.\]

\(M_{\xi, \delta}^{-1}\), which reduces the computational cost, is applied to all parts of this algorithm, \(\beta\) samples are extracted from the posterior using fast sampling(Bhattacharya et al.,2016) as follows.

\[u \sim N_{p}(0, \xi^{-1}D),\quad f \sim N_{N}(0, I_{N}), \\ v = Xu + f,\quad v^{\star} = M_{\xi, \delta}^{-1}(y/\sigma - v), \\ \beta = \sigma(u + \xi^{-1}D_{\delta}X^{T}v^{\star}).\]

The above changed algorithm is implemented in the approx_horseshoe and the mapprox_horseshoe functions of this package. The approx_horseshoe uses a fixed \(\delta\), while the mapprox_horseshoe uses an algorithm that updates \(\delta\) using adaptive probability.

The modified approximate algorithm developed in this package estimates new threshold through updated shrinkage parameters and adds an adaptive probability algorithm that updates the threshold. the process of updating a new threshold is added by applying the properties of the shrinkage weight \(k_{j},\ j=1,2,...,p\) proposed by Piironen and Vehtari (2017). In the prior of \(\beta_{j} \sim N(0, \sigma^{2}\tau^{2}\lambda_{j}^{2}) = N(0, \sigma^{2}\xi^{-1}\eta_{j}^{-1})\), the variable \(m_{eff}\) is defined as follows.

\[k_{j} = 1/\left(1+n\xi^{-1}s_{j}^{2}\eta_{j}^{-1} \right), \quad j=1,2,...,p, \\ m_{eff} = \sum_{j=1}^{p}{\left(1-k_{j} \right)}.\]

\(s_{j},\ j=1,2,...,p\) are the diagonal components of \(X^{T}X\). For the zero components of \(\beta\), \(k_{j}\) is derived close to 1, and nonzero’s \(k_{j}\) is derived close to 0, so the variable \(m_{eff}\) is called the effective number of nonzero coefficients. In this algorithm, the threshold \(\delta\) is updated to set \(s_{\delta} = ceiling(m_{eff})\).

Adaptive probability is defined to satisfy Theorem 5(diminishing adaptation condition) of Roberts and Rosenthal (2007). at \(T\)th iteration,

\[p(T) = exp[p_{0} + p_{1}T],\quad p_{1} < 0, \quad u \sim U(0, 1), \\ if\ u < p(T),\ update\ \delta\ so\ that\ s_{\delta} = ceiling(m_{eff}).\]

The default is \(p_{0} = 0\), \(p_{1} = -4.6 \times 10^{-4}\), and under this condition, \(p(10000) < 0.01\) is satisfied.

```
# approx_horseshoe with fixed default threshold.
approx_horseshoe_result <- approx_horseshoe(X, y, burn = 0, iter = 500,
auto.threshold = FALSE)
#> You chose FALSE for the auto.threshold argument. and since threshold = 0, set it to the default value of 0.002.
# modified approx_horseshoe with adaptive probability algorithm.
mapprox_horseshoe_result <- approx_horseshoe(X, y, burn = 0, iter = 500)
df2 <- data.frame(index = 1:100,
approx_BetaHat = approx_horseshoe_result$BetaHat[1:100],
approx_LeftCI = approx_horseshoe_result$LeftCI[1:100],
approx_RightCI = approx_horseshoe_result$RightCI[1:100],
mapprox_BetaHat = mapprox_horseshoe_result$BetaHat[1:100],
mapprox_LeftCI = mapprox_horseshoe_result$LeftCI[1:100],
mapprox_RightCI = mapprox_horseshoe_result$RightCI[1:100],
true_beta = true_beta[1:100])
# Estimation results of the approx_horseshoe.
ggplot(data = df2, aes(x = index, y = true_beta)) +
geom_point(size = 2) +
geom_point(aes(x = index, y = approx_BetaHat), size = 2, col = "red") +
geom_errorbar(aes(ymin = approx_LeftCI,
ymax = approx_RightCI), width = .1, col = "red") +
labs(title = "95% Credible intervals of the approx_horseshoe", y = "beta")
```

```
# Estimation results of the mapprox_horseshoe.
ggplot(data = df2, aes(x = index, y = true_beta)) +
geom_point(size = 2) +
geom_point(aes(x = index, y = mapprox_BetaHat),
size = 2, col = "red") +
geom_errorbar(aes(ymin = mapprox_LeftCI,
ymax = mapprox_RightCI),
width = .1, col = "red") +
labs(title = "95% Credible intervals of the modified_approx_horseshoe",
y = "beta")
```

In the case of exact_horseshoe, since the \(D\) is used as is, it can always be thought of as \(length(S) = 500\). On the other hand, since the method of setting the threshold is different in approx_horseshoe and modified approx_horseshoe, a difference occurs in \(length(S) = s_{\delta}\), which causes a difference in the calculation speed of the algorithm.

```
exact_activeset <- rep(p, 500)
approx_activeset <- apply(approx_horseshoe_result$ActiveSet, MARGIN = 1, sum)
mapprox_activeset <- apply(mapprox_horseshoe_result$ActiveSet, MARGIN = 1, sum)
# active set plot
ggplot(data = data.frame(X = 1:500,
exact_activeset = exact_activeset,
approx_activeset = approx_activeset,
mapprox_activeset = mapprox_activeset)) +
geom_line(mapping = aes(x = X, y = exact_activeset,
color = "exact")) +
geom_line(mapping = aes(x = X, y = approx_activeset,
color = "approx"),
alpha = 0.5) +
geom_line(mapping = aes(x = X, y = mapprox_activeset,
color = "modified_approx"),
alpha = 0.5) +
scale_color_manual(name = "algorithm",
values = c("black", "red", "blue"),
breaks = c("exact", "approx", "modified_approx"),
labels = c("exact", "approx", "modified_approx"))
```

Bhattacharya, A., Chakraborty, A., & Mallick, B. K. (2016). Fast sampling with Gaussian scale mixture priors in high-dimensional regression. Biometrika, asw042.

Johndrow, J., Orenstein, P., & Bhattacharya, A. (2020). Scalable Approximate MCMC Algorithms for the Horseshoe Prior. In Journal of Machine Learning Research (Vol. 21).

Piironen, J., & Vehtari, A. (2017). Sparsity information and regularization in the horseshoe and other shrinkage priors. Electronic Journal of Statistics, 11, 5018-5051.

Roberts G, Rosenthal J. Coupling and ergodicity of adaptive Markov chain Monte Carlo algorithms. J Appl Prob. 2007;44:458–475.