niw.post: Random draws from the posterior distribution with...

In nicheROVER: Niche Region and Niche Overlap Metrics for Multidimensional Ecological Niches

Random draws from the posterior distribution with Normal-Inverse-Wishart (NIW) prior.

Description

Given iid d-dimensional niche indicators X = (X_1,\ldots,X_N) with X_i \sim N(\mu, \Sigma), this function generates random draws from p(\mu,\Sigma | X) for the Normal-Inverse-Wishart (NIW) prior.

Usage

niw.post(nsamples, X, lambda, kappa, Psi, nu)

Arguments

nsamples	The number of posterior draws.
X	A data matrix with observations along the rows.
lambda	Location parameter. See 'Details'.
kappa	Scale parameter. Defaults to kappa = 0. See 'Details'.
Psi	Scale matrix. Defaults to Psi = 0. See Details.
nu	Degrees of freedom. Defaults to nu = ncol(X)+1. See Details.

Details

The NIW distribution p(\mu, \Sigma | \lambda, \kappa, \Psi, \nu) is defined as

\Sigma \sim W^{-1}(\Psi, \nu), \quad \mu | \Sigma \sim N(\lambda, \Sigma/\kappa).

The default value kappa = 0 uses the Lebesque prior on \mu: p(\mu) \propto 1.

The default value Psi = 0 uses the scale-invariant prior on \Sigma: p(\Sigma) \propto |\Sigma|^{-(\nu+d+1)/2}.

The default value nu = ncol(X)+1 for kappa = 0 and Psi = 0 makes E[\mu|X]=\code{colMeans(X)} and E[\Sigma | X]=\code{var(X)}.

Value

Returns a list with elements mu and Sigma of sizes c(nsamples, length(lambda)) and c(dim(Psi), nsamples).

See Also

rniw(), niiw.post().

Examples

# compare the default non-informative prior to an arbitrary informative prior
# for simulated data

# simulate normal data with mean and variance (mu0, Sigma0)
d <- 4
mu0 <- rnorm(d)
Sigma0 <- matrix(rnorm(d^2), d, d)
Sigma0 <- Sigma0 %*% t(Sigma0)
N <- 1e2
X <- matrix(rnorm(N*d), N, d) # iid N(0,1)
X <- t(t(X %*% chol(Sigma0)) + mu0) # each row is N(mu0, Sigma)

# informative prior parameters
lambda <- rnorm(d)
kappa <- 20
Psi <- crossprod(matrix(rnorm(d^2), d, d))
nu <- 5

# iid draws from informative prior pi(mu, Sigma)
nsamples <- 2e3
siw0 <- rniw(nsamples, lambda, kappa, Psi, nu)

# iid draws from posterior p(mu, Sigma | X) with informative prior
siw1 <- niw.post(nsamples, X, lambda, kappa, Psi, nu)

# iid draws from posterior p(mu, Sigma | X) with default noninformative prior
siw2 <- niw.post(nsamples, X)

# compare

# prior and posterior densities of mu
clrs <- c("orange", "red", "blue", "black")
ii <- 1
par(mar = c(4.2, 4.2, 2, 1)+.1)
niche.par.plot(list(siw0, siw1, siw2), col = clrs[1:3],
               plot.index = ii, ylab = "Density")
abline(v = mu0[ii], col = clrs[4]) # true value of mu
legend(x = "topright",
       legend = c(parse(text = paste0("pi(mu[", ii, "])")),
                  parse(text = paste0("p(mu[", ii, "]*\" | \"*X)*\", Informative Prior\"")),
                  parse(text = paste0("p(mu[", ii, "]*\" | \"*X)*\", Noninformative Prior\"")),
                  parse(text = paste0("\"True value of \"*mu[", ii, "]"))),
       fill = clrs)

# prior and posterior densities of Sigma
ii <- 1
jj <- 2
par(mar = c(4.2, 4.2, 2, 1)+.1)
niche.par.plot(list(siw0, siw1, siw2), col = clrs[1:3],
               plot.index = c(ii,jj), ylab = "Density")
abline(v = Sigma0[ii,jj], col = clrs[4])
legend(x = "topright",
       legend = c(parse(text = paste0("pi(Sigma[", ii, "*", jj, "])")),
                  parse(text = paste0("p(Sigma[", ii, "*", jj,
                                      "]*\" | \"*X)*\", Informative Prior\"")),
                  parse(text = paste0("p(Sigma[", ii, "*", jj,
                                      "]*\" | \"*X)*\", Noninformative Prior\"")),
                  parse(text = paste0("\"True value of \"*Sigma[", ii, "*", jj, "]"))),
       fill = clrs)

nicheROVER documentation built on Oct. 13, 2023, 5:10 p.m.