高校数学の質問スレ（医者・東大卒専用） Part438 (991ﾚｽ)
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842: １３２人目の素数さん [sage] 05/24(土)02:35 ID:VetM3rz7(1/5)
LearnBayes::beta.selectをoptimを使って算出

beta.optim <- function(x1, p1, x2, p2, verbose = TRUE) {
# -------------------------
# モーメント近似による初期値推定
# -------------------------
mu0 <- (x1 + x2) / 2 # 仮の平均
sigma2 <- ((x2 - x1) / 4)^2 # 仮の分散（中間50%幅から）
v <- mu0 * (1 - mu0) / sigma2 - 1
a0 <- mu0 * v
b0 <- (1 - mu0) * v
start <- c(a0, b0)

# -------------------------
# 最適化対象の誤差関数定義
# -------------------------
objective <- function(params) {
a <- params[1]
b <- params[2]
# pbeta による累積確率との差を2乗誤差で評価
err1 <- pbeta(x1, a, b) - p1
err2 <- pbeta(x2, a, b) - p2
return(err1^2 + err2^2)
}

# -------------------------
# 最適化（境界付き）
# -------------------------
result <- optim(
par = start,
fn = objective,
method = "L-BFGS-B",
lower = c(1e-4, 1e-4) # ベータ分布は a, b > 0
)

# -------------------------
# 結果の取り出し
# -------------------------
a_hat <- result$par[1]
b_hat <- result$par[2]

if (verbose) {
cat("推定されたパラメータ: a =", round(a_hat, 4),
", b =", round(b_hat, 4), "\n")
cat("pbeta(x1) =", round(pbeta(x1, a_hat, b_hat), 4),
"（目標:", p1, "）\n")
cat("pbeta(x2) =", round(pbeta(x2, a_hat, b_hat), 4),
"（目標:", p2, "）\n")
}

# -------------------------
# 結果を返す
# -------------------------
return(list(a = a_hat, b = b_hat, ss_value = result$value))
}

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843: １３２人目の素数さん [sage] 05/24(土)08:17 ID:VetM3rz7(2/5)
library(rjags)

# Fit a Bayesian logistic regression model using JAGS and return predictions and posterior summaries
fit_bayesian_logistic_jags <- function(data, formula, newdata,
n.chains = 3, n.iter = 5000, n.burnin = 1000) {
# Extract response variable name from the formula
response_var <- all.vars(formula)[1]
y <- data[[response_var]]

# Convert factor response to binary numeric (0/1)
if (is.factor(y)) y <- as.numeric(y) - 1
y <- as.numeric(y)

# Construct design matrices for training and new data
X <- model.matrix(formula, data)
new_X <- model.matrix(delete.response(terms(formula)), newdata)

# Prepare data list for JAGS
jags_data <- list(
y = y,
X = X,
n = nrow(X),
p = ncol(X),
new_X = new_X,
scale_beta = rep(2.5, ncol(X)) # Prior scale for each coefficient
)

# Define the JAGS model
model_string <- "
model {
for (j in 1:p) {
beta[j] ~ dt(0, 1 / pow(scale_beta[j], 2), 1)
}

for (i in 1:n) {
logit_p[i] <- inprod(X[i,], beta[])
y[i] ~ dbern(1 / (1 + exp(-logit_p[i])))
}

new_logit <- inprod(new_X[1,], beta[])
new_p <- 1 / (1 + exp(-new_logit))
}
"

# Initialize and run the JAGS model
model <- jags.model(textConnection(model_string), data = jags_data,
n.chains = n.chains, quiet = TRUE)
update(model, n.burnin)

# Draw posterior samples
samples <- coda.samples(model, c("beta", "new_p"), n.iter - n.burnin)
mat <- as.matrix(samples)

# Return results
list(
model = samples,
predicted_prob = mean(mat[, "new_p"]),
summary = summary(samples)
)
}

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844: １３２人目の素数さん [sage] 05/24(土)08:18 ID:VetM3rz7(3/5)
# Example data
data <- data.frame(
donation = c(0, 1000, 2000, 0, 3000, 0, 4000, 0, 5000, 0),
score = c(90, 40, 35, 88, 30, 85, 25, 92, 20, 89),
parent = c(0, 1, 1, 0, 1, 0, 1, 0, 1, 0),
admission = as.factor(c(0, 1, 1, 0, 1, 0, 1, 0, 1, 0))
)

# New observation to predict
newdata <- data.frame(
donation = 2500,
score = 40,
parent = 1
)

# Fit model and obtain results
set.seed(123)
result <- fit_bayesian_logistic_jags(
data = data,
formula = admission ~ donation + score + parent,
newdata = newdata
)

# Extract variable names including intercept
var_names <- colnames(model.matrix(admission ~ donation + score + parent, data))

# Extract beta coefficient summaries
beta_stats <- result$summary$statistics[grep("^beta\\[", rownames(result$summary$statistics)), c("Mean", "SD")]
beta_quants <- result$summary$quantiles[grep("^beta\\[", rownames(result$summary$quantiles)), c("2.5%", "97.5%")]

# Rename row names using variable names
rownames(beta_stats) <- var_names
rownames(beta_quants) <- var_names

# Display results
print(beta_stats)
print(beta_quants)
cat("Predicted probability:", round(result$predicted_prob, 3), "\n")

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845: １３２人目の素数さん [sage] 05/24(土)08:37 ID:VetM3rz7(4/5)
library(rjags)

# Fit a Bayesian logistic regression model using JAGS and return predictions and posterior summaries
fit_bayesian_logistic_jags <- function(data, formula, newdata,
n.chains = 3, n.iter = 5000, n.burnin = 1000) {
# Extract response variable name from the formula
response_var <- all.vars(formula)[1]
y <- data[[response_var]]

# Convert factor response to binary numeric (0/1)
if (is.factor(y)) y <- as.numeric(y) - 1
y <- as.numeric(y)

# Construct design matrices for training and new data
X <- model.matrix(formula, data)
new_X <- model.matrix(delete.response(terms(formula)), newdata)

# Prepare data list for JAGS
jags_data <- list(
y = y,
X = X,
n = nrow(X),
p = ncol(X),
new_X = new_X,
scale_beta = rep(2.5, ncol(X)) # Prior scale for each coefficient
)

# Define the JAGS model
model_string <- "
model {
for (j in 1:p) {
beta[j] ~ dt(0, 1 / pow(scale_beta[j], 2), 1)
}

for (i in 1:n) {
logit_p[i] <- inprod(X[i,], beta[])
y[i] ~ dbern(1 / (1 + exp(-logit_p[i])))
}

new_logit <- inprod(new_X[1,], beta[])
new_p <- 1 / (1 + exp(-new_logit))
}
"

# Initialize and run the JAGS model
model <- jags.model(textConnection(model_string), data = jags_data,
n.chains = n.chains, quiet = TRUE)
update(model, n.burnin)

# Draw posterior samples
samples <- coda.samples(model, c("beta", "new_p"), n.iter - n.burnin)
mat <- as.matrix(samples)

# Return results
list(
model = samples,
predicted_prob = mean(mat[, "new_p"]),
summary = summary(samples)
)
}

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846: １３２人目の素数さん [sage] 05/24(土)21:16 ID:VetM3rz7(5/5)
# dbeta(L,a,b) == dbbeta(U,a,b)
# Solve[L^(a-1)(1-L)^(b-1)==U^(a-1)(1-U)^(b-1), b]

L=1/7
U=1/5
credMass = 0.95
f = function(a) 1 + ((a - 1) * log(U / L)) / log((1 - L) / (1 - U))
g = function(a) pbeta(U,a,f(a)) - pbeta(L,a,f(a)) - credMass
(re=uniroot(g,c(1,1e5)))
curve(g(x),1,150,bty="l") ; abline(h=0,lty=3)
c(re$root,f(re$root))

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