我有以下多项式回归模型：
图片版本
乳胶版
$Y_i | \mu_i, \sigma^2 \sim \text{正常}(\mu_i, \sigma^2), i = 1, \dots, n \ \text{独立}$
$\mu_i = \alpha + \beta_1 x_{i1} + \beta_2 x_{i2} + \beta_3 x_{i1}^2 + \beta_4 x_{i2}^2 + \beta_5 x_{i1} x_{i2}$
$\alpha \sim \text{一些合适的先验}$
$\beta_1, \dots, \beta_5 \sim \text{一些合适的先验}$
$\sigma^2 \sim \text{一些合适的先验}$
我想将样本大小和 $y_i$、$x_{i1}$ 和 $x_{i2}$ 上的观察向量作为输入。代码如下：
data{
int<lower=1> n;
vector[n] x1;
vector[n] x2;
vector[n] y;
}
我想对两个输入变量进行标准化（中心化和缩放）以获得标准化的回归变量x1_std和x2_std. 代码在transformed data块中，如下所示：
transformed data{
real bar_x1;
real x1_sd;
vector[n] x1_std;
real bar_x2;
real x2_sd;
vector[n] x2_std;
real y_sd;
bar_x1 = mean(x1);
x1_sd = sd(x1);
x1_std = (x1 - bar_x1)/x1_sd; // centered and scaled
bar_x2 = mean(x2);
x2_sd = sd(x2);
x2_std = (x2 - bar_x2)/x2_sd; // centered and scaled
y_sd = sd(y);
}
然后，我想使用标准化回归变量拟合上述多项式回归模型，并在原始和标准化尺度上返回回归参数 $\alpha$、$\beta_1$ 和 $\dots、\beta_5$ 的估计值。
基于此，如果我没记错的话，标准化参数到原始尺度的转换公式如下：
图片版本
乳胶版
$\alpha = \tilde{\alpha} - \dfrac{\gamma_1}{s_1}\bar{x}_1 - \dfrac{\gamma_2}{s_2}\bar{x}_2 + \dfrac{\gamma_3}{ s_1^2}\bar{x}_1^2 + \dfrac{\gamma_4}{s_2^2}\bar{x}_2^2 + \dfrac{\gamma_5}{s_1 s_2}\bar{x}_1\酒吧{x}_2$
$\beta_1 = \left( \dfrac{\gamma_1}{s_1} - 2\dfrac{\gamma_3}{s_1^2}\bar{x}_1 - \dfrac{\gamma_5}{s_1 s_2}\bar{x }_2 \右）$
$\beta_2 = \left( \dfrac{\gamma_2}{s_2} - 2\dfrac{\gamma_4}{s_2^2}\bar{x}_2 - \dfrac{\gamma_5}{s_1 s_2}\bar{x }_1 \右）$
$\beta_3 = \dfrac{\gamma_3}{s_1^2}$
$\beta_4 = \dfrac{\gamma_4}{s_2^2}$
$\beta_5 = \dfrac{\gamma_5}{s_1 s_2}$
实现这一点的代码包含在generated quantities块中，如下所示：
alpha = alpha_std - beta1_std*bar_x1/x1_sd - beta2_std*bar_x2/x2_sd
+ (beta3_std*bar_x1^2)/x1_sd^2 + (beta4_std*bar_x2^2)/x2_sd^2
+ (beta5_std*bar_x2*bar_x1)/(x1_sd*x2_sd);
beta1 = beta1_std/x1_sd - 2*beta3_std*bar_x1/x1_sd^2
- beta5_std*bar_x2/(x1_sd*x2_sd);
beta2 = beta2_std/x2_sd - 2*beta4_std*bar_x2/x2_sd^2
- beta5_std*bar_x1/(x1_sd*x2_sd);
beta3 = beta3_std/x1_sd^2;
beta4 = beta4_std/x2_sd^2;
beta5 = beta5_std/(x1_sd*x2_sd);
我的整个模型如下：
data{
int<lower=1> n;
vector[n] x1;
vector[n] x2;
vector[n] y;
}
transformed data{
real bar_x1;
real x1_sd;
vector[n] x1_std;
real bar_x2;
real x2_sd;
vector[n] x2_std;
real y_sd;
bar_x1 = mean(x1);
x1_sd = sd(x1);
x1_std = (x1 - bar_x1)/x1_sd; // centered and scaled
bar_x2 = mean(x2);
x2_sd = sd(x2);
x2_std = (x2 - bar_x2)/x2_sd; // centered and scaled
y_sd = sd(y);
}
parameters{
real<lower=0> sigma;
real alpha_std;
real beta1_std;
real beta2_std;
real beta3_std;
real beta4_std;
real beta5_std;
}
transformed parameters {
real mu[n];
for(i in 1:n) {
mu[i] = alpha_std + beta1_std*x1_std[i]
+ beta2_std*x2_std[i] + beta3_std*x1_std[i]^2
+ beta4_std*x2_std[i]^2 + beta5_std*x1_std[i]*x2_std[i];
}
}
model{
alpha_std ~ normal(0, 10);
beta1_std ~ normal(0, 2.5);
beta2_std ~ normal(0, 2.5);
beta3_std ~ normal(0, 2.5);
beta4_std ~ normal(0, 2.5);
beta5_std ~ normal(0, 2.5);
sigma ~ exponential(1 / y_sd);
y ~ normal(mu, sigma);
}
generated quantities {
real alpha;
real beta1;
real beta2;
real beta3;
real beta4;
real beta5;
alpha = alpha_std - beta1_std*bar_x1/x1_sd - beta2_std*bar_x2/x2_sd
+ (beta3_std*bar_x1^2)/x1_sd^2 + (beta4_std*bar_x2^2)/x2_sd^2
+ (beta5_std*bar_x2*bar_x1)/(x1_sd*x2_sd);
beta1 = beta1_std/x1_sd - 2*beta3_std*bar_x1/x1_sd^2
- beta5_std*bar_x2/(x1_sd*x2_sd);
beta2 = beta2_std/x2_sd - 2*beta4_std*bar_x2/x2_sd^2
- beta5_std*bar_x1/(x1_sd*x2_sd);
beta3 = beta3_std/x1_sd^2;
beta4 = beta4_std/x2_sd^2;
beta5 = beta5_std/(x1_sd*x2_sd);
}
我正在使用hillsR 包中的数据集MASS：
library(MASS)
hills[18, 3] <- 18.65 # Fixing transcription error
x1 <- hills$dist
x2 <- hills$climb
y <- hills$time
n <- length(x1)
data.in <- list(x1 = x1, x2 = x2, y = y, n = n)
model.fit <- sampling(example, data.in)
现在我输出标准化的 ( alpha_std, beta1_std, beta2_std, beta3_std, beta4_std, beta5_std) 和原始尺度 ( alpha, beta1, beta2, beta3, beta4, beta5) 回归参数：
print(model.fit, pars = c("alpha_std", "alpha", "beta1_std", "beta2_std", "beta3_std", "beta4_std", "beta5_std", "beta1", "beta2", "beta3", "beta4", "beta5", "sigma"), probs = c(0.05, 0.5, 0.95), digits = 5)
我做对了吗？我还对数学进行了两次和三次检查，所以我认为它应该是正确的。尽管如此，我很紧张的一件事beta4是 0.00000。这是否表明我犯了错误？正如我所说，我已经检查了我所有的代码和数学，所以，据我所知，一切似乎都很好。