我正在尝试在方形区域上整合独立的二元正态分布。数值积分与蒙特卡罗模拟不匹配。这里出了什么问题?
import numpy as np
from scipy import integrate
from scipy.stats import multivariate_normal
sigmaX = 0.5
sigmaY = 0.8
region = 1.0 # Region of interest is a unit square centered at the mean (0,0)
# Numerical integration for answer:
def pdf(x,y):
return multivariate_normal.pdf([x,y], mean=[0,0], cov=[[sigmaX, 0], [0, sigmaY]])
probability, err = integrate.nquad(pdf, [[-region/2.0, region/2.0], [-region/2.0, region/2.0]])
# Monte Carlo simulation for answer:
simulations = 1_000_000
X = np.random.normal(scale=sigmaX, size=simulations)
Y = np.random.normal(scale=sigmaY, size=simulations)
hits = sum(1 for s in range(simulations) if ((abs(X[s]) < region/2.0) and (abs(Y[s]) < region/2.0)))/simulations
print(f'Numerical integration gives probability {probability:.1%}\n'
f'Monte Carlo gives probability {hits:.1%}')
输出:
数值积分给出的概率为 22.1%
蒙特卡洛给出的概率为 31.9%
协方差矩阵的对角线是方差,而
scale
的np.random.normal
参数是*标准差。修复计算的一种方法是将 cov
函数的 pdf
参数更改为
cov=[[sigmaX**2, 0], [0, sigmaY**2]]