使用smf.ols的{{1}}和sm.OLS函数，我在系数值和系数误差方面存在差异。即使在数学上，它们也应该是相同的回归公式并给出相同的结果。
我已经完成了我的问题的100％可复制示例，可以从此处下载数据框df：https://drive.google.com/drive/folders/1i67wztkrAeEZH2tv2hyOlgxG7N80V3pI?usp=sharing
statsmodelslm1的结果是：
# First we load the libraries:
import statsmodels.api as sm
import statsmodels.formula.api as smf
import random
import pandas as pd
# We define a specific seed to have the same results:
random.seed(1234)
# Now we read the data that can be downloaded from Google Drive link provided above:
df = pd.read_csv("/Users/user/Documents/example/cars.csv", sep = "|")
# We create the linear regression:
lm1 = smf.ols('price ~ make + fuel_system + engine_type + num_of_doors + bore + compression_ratio + height + peak_rpm + 1', data = df)
# We see the results:
lm1.fit().summary()
OLS Regression Results
==============================================================================
Dep. Variable: price R-squared: 0.894
Model: OLS Adj. R-squared: 0.868
Method: Least Squares F-statistic: 35.54
Date: Mon, 18 Feb 2019 Prob (F-statistic): 5.24e-62
Time: 17:19:14 Log-Likelihood: -1899.7
No. Observations: 205 AIC: 3879.
Df Residuals: 165 BIC: 4012.
Df Model: 39
Covariance Type: nonrobust
=========================================================================================
coef std err t P>|t| [0.025 0.975]
-----------------------------------------------------------------------------------------
Intercept 1.592e+04 1.21e+04 1.320 0.189 -7898.396 3.97e+04
make[T.audi] 6519.7045 2371.807 2.749 0.007 1836.700 1.12e+04
make[T.bmw] 1.427e+04 2292.551 6.223 0.000 9740.771 1.88e+04
make[T.chevrolet] -571.8236 2860.026 -0.200 0.842 -6218.788 5075.141
make[T.dodge] -1186.3430 2261.240 -0.525 0.601 -5651.039 3278.353
make[T.honda] 2779.6496 2891.626 0.961 0.338 -2929.709 8489.009
make[T.isuzu] 3098.9677 2592.645 1.195 0.234 -2020.069 8218.004
make[T.jaguar] 1.752e+04 2416.313 7.252 0.000 1.28e+04 2.23e+04
make[T.mazda] 306.6568 2134.567 0.144 0.886 -3907.929 4521.243
make[T.mercedes-benz] 1.698e+04 2320.871 7.318 0.000 1.24e+04 2.16e+04
make[T.mercury] 2958.1002 3605.739 0.820 0.413 -4161.236 1.01e+04
make[T.mitsubishi] -1188.8337 2284.697 -0.520 0.604 -5699.844 3322.176
make[T.nissan] -1211.5463 2073.422 -0.584 0.560 -5305.405 2882.312
make[T.peugot] 3057.0217 4255.809 0.718 0.474 -5345.841 1.15e+04
make[T.plymouth] -894.5921 2332.746 -0.383 0.702 -5500.473 3711.289
make[T.porsche] 9558.8747 3688.038 2.592 0.010 2277.044 1.68e+04
make[T.renault] -2124.9722 2847.536 -0.746 0.457 -7747.277 3497.333
make[T.saab] 3490.5333 2319.189 1.505 0.134 -1088.579 8069.645
make[T.subaru] -1.636e+04 4002.796 -4.087 0.000 -2.43e+04 -8456.659
make[T.toyota] -770.9677 1911.754 -0.403 0.687 -4545.623 3003.688
make[T.volkswagen] 406.9179 2219.714 0.183 0.855 -3975.788 4789.623
make[T.volvo] 5433.7129 2397.030 2.267 0.025 700.907 1.02e+04
fuel_system[T.2bbl] 2142.1594 2232.214 0.960 0.339 -2265.226 6549.545
fuel_system[T.4bbl] 464.1109 3999.976 0.116 0.908 -7433.624 8361.846
fuel_system[T.idi] 1.991e+04 6622.812 3.007 0.003 6837.439 3.3e+04
fuel_system[T.mfi] 3716.5201 3936.805 0.944 0.347 -4056.488 1.15e+04
fuel_system[T.mpfi] 3964.1109 2267.538 1.748 0.082 -513.019 8441.241
fuel_system[T.spdi] 3240.0003 2719.925 1.191 0.235 -2130.344 8610.344
fuel_system[T.spfi] 932.1959 4019.476 0.232 0.817 -7004.041 8868.433
engine_type[T.dohcv] -1.208e+04 4205.826 -2.872 0.005 -2.04e+04 -3773.504
engine_type[T.l] -4833.9860 3763.812 -1.284 0.201 -1.23e+04 2597.456
engine_type[T.ohc] -4038.8848 1213.598 -3.328 0.001 -6435.067 -1642.702
engine_type[T.ohcf] 9618.9281 3504.600 2.745 0.007 2699.286 1.65e+04
engine_type[T.ohcv] 3051.7629 1445.185 2.112 0.036 198.323 5905.203
engine_type[T.rotor] 1403.9928 3217.402 0.436 0.663 -4948.593 7756.579
num_of_doors[T.two] -419.9640 521.754 -0.805 0.422 -1450.139 610.211
bore 3993.4308 1373.487 2.908 0.004 1281.556 6705.306
compression_ratio -1200.5665 460.681 -2.606 0.010 -2110.156 -290.977
height -80.7141 146.219 -0.552 0.582 -369.417 207.988
peak_rpm -0.5903 0.790 -0.747 0.456 -2.150 0.970
==============================================================================
Omnibus: 65.777 Durbin-Watson: 1.217
Prob(Omnibus): 0.000 Jarque-Bera (JB): 399.594
Skew: 1.059 Prob(JB): 1.70e-87
Kurtosis: 9.504 Cond. No. 3.26e+05
==============================================================================
Warnings:
[1] Standard Errors assume that the covariance matrix of the errors is correctly specified.
[2] The condition number is large, 3.26e+05. This might indicate that there are
strong multicollinearity or other numerical problems.
"""
lm2的结果是：
# We define a specific seed to have the same results:
random.seed(1234)
# First we check what `object` type variables we have in our dataset:
df.dtypes
# We create a list where we save the `object` type variables names:
object = ['make',
'fuel_system',
'engine_type',
'num_of_doors'
]
# Now we convert those object variables to numeric with get_dummies function to have 1 unique numeric dataframe:
df_num = pd.get_dummies(df, columns = object)
# We ensure the dataframe is numeric casting all values to float64:
df_num = df_num[df_num.columns].apply(pd.to_numeric, errors='coerce', axis = 1)
# We define the predictive variables dataset:
X = df_num.drop('price', axis = 1)
# We define the response variable values:
y = df_num.price.values
# We add a constant as we did in the previous example (adding "+1" to Patsy):
Xc = sm.add_constant(X) # Adds a constant to the model
# We create the linear model and obtain results:
lm2 = sm.OLS(y, Xc)
lm2.fit().summary()
我们可以看到，像 OLS Regression Results
==============================================================================
Dep. Variable: y R-squared: 0.894
Model: OLS Adj. R-squared: 0.868
Method: Least Squares F-statistic: 35.54
Date: Mon, 18 Feb 2019 Prob (F-statistic): 5.24e-62
Time: 17:28:16 Log-Likelihood: -1899.7
No. Observations: 205 AIC: 3879.
Df Residuals: 165 BIC: 4012.
Df Model: 39
Covariance Type: nonrobust
======================================================================================
coef std err t P>|t| [0.025 0.975]
--------------------------------------------------------------------------------------
const 1.205e+04 6811.094 1.769 0.079 -1398.490 2.55e+04
bore 3993.4308 1373.487 2.908 0.004 1281.556 6705.306
compression_ratio -1200.5665 460.681 -2.606 0.010 -2110.156 -290.977
height -80.7141 146.219 -0.552 0.582 -369.417 207.988
peak_rpm -0.5903 0.790 -0.747 0.456 -2.150 0.970
make_alfa-romero -2273.9631 1865.185 -1.219 0.225 -5956.669 1408.743
make_audi 4245.7414 1324.140 3.206 0.002 1631.299 6860.184
make_bmw 1.199e+04 1232.635 9.730 0.000 9559.555 1.44e+04
make_chevrolet -2845.7867 1976.730 -1.440 0.152 -6748.733 1057.160
make_dodge -3460.3061 1170.966 -2.955 0.004 -5772.315 -1148.297
make_honda 505.6865 2049.865 0.247 0.805 -3541.661 4553.034
make_isuzu 825.0045 1706.160 0.484 0.629 -2543.716 4193.725
make_jaguar 1.525e+04 1903.813 8.010 0.000 1.15e+04 1.9e+04
make_mazda -1967.3063 982.179 -2.003 0.047 -3906.564 -28.048
make_mercedes-benz 1.471e+04 1423.004 10.338 0.000 1.19e+04 1.75e+04
make_mercury 684.1370 2913.361 0.235 0.815 -5068.136 6436.410
make_mitsubishi -3462.7968 1221.018 -2.836 0.005 -5873.631 -1051.963
make_nissan -3485.5094 946.316 -3.683 0.000 -5353.958 -1617.060
make_peugot 783.0586 3513.296 0.223 0.824 -6153.754 7719.871
make_plymouth -3168.5552 1293.376 -2.450 0.015 -5722.256 -614.854
make_porsche 7284.9115 2853.174 2.553 0.012 1651.475 1.29e+04
make_renault -4398.9354 2037.945 -2.159 0.032 -8422.747 -375.124
make_saab 1216.5702 1487.192 0.818 0.415 -1719.810 4152.950
make_subaru -1.863e+04 3263.524 -5.710 0.000 -2.51e+04 -1.22e+04
make_toyota -3044.9308 776.059 -3.924 0.000 -4577.218 -1512.644
make_volkswagen -1867.0452 1170.975 -1.594 0.113 -4179.072 444.981
make_volvo 3159.7498 1327.405 2.380 0.018 538.862 5780.638
fuel_system_1bbl -2790.4092 2230.161 -1.251 0.213 -7193.740 1612.922
fuel_system_2bbl -648.2498 1094.525 -0.592 0.554 -2809.330 1512.830
fuel_system_4bbl -2326.2983 3094.703 -0.752 0.453 -8436.621 3784.024
fuel_system_idi 1.712e+04 6154.806 2.782 0.006 4971.083 2.93e+04
fuel_system_mfi 926.1109 3063.134 0.302 0.763 -5121.881 6974.102
fuel_system_mpfi 1173.7017 1186.125 0.990 0.324 -1168.238 3515.642
fuel_system_spdi 449.5911 1827.318 0.246 0.806 -3158.349 4057.531
fuel_system_spfi -1858.2133 3111.596 -0.597 0.551 -8001.891 4285.464
engine_type_dohc 2703.6445 1803.080 1.499 0.136 -856.440 6263.729
engine_type_dohcv -9374.0342 3504.717 -2.675 0.008 -1.63e+04 -2454.161
engine_type_l -2130.3416 3357.283 -0.635 0.527 -8759.115 4498.431
engine_type_ohc -1335.2404 1454.047 -0.918 0.360 -4206.177 1535.696
engine_type_ohcf 1.232e+04 2850.883 4.322 0.000 6693.659 1.8e+04
engine_type_ohcv 5755.4074 1669.627 3.447 0.001 2458.820 9051.995
engine_type_rotor 4107.6373 3032.223 1.355 0.177 -1879.323 1.01e+04
num_of_doors_four 6234.8048 3491.722 1.786 0.076 -659.410 1.31e+04
num_of_doors_two 5814.8408 3337.588 1.742 0.083 -775.045 1.24e+04
==============================================================================
Omnibus: 65.777 Durbin-Watson: 1.217
Prob(Omnibus): 0.000 Jarque-Bera (JB): 399.594
Skew: 1.059 Prob(JB): 1.70e-87
Kurtosis: 9.504 Cond. No. 1.01e+16
==============================================================================
Warnings:
[1] Standard Errors assume that the covariance matrix of the errors is correctly specified.
[2] The smallest eigenvalue is 5.38e-23. This might indicate that there are
strong multicollinearity problems or that the design matrix is singular.
"""
这样的变量具有相同的系数。不过，有些人则没有（变量height的{{1}}级别，isuzu的{{1}}级别或make的级别等等）。两个输出的结果不应该相同吗？我在这里想念什么或做错什么了吗？
预先感谢您的帮助。
P.D。正如@sukhbinder所阐明的，即使使用没有独立的Patsy公式 术语（在公式中输入“ -1”，因为Patsy通过 默认值）并从虚拟公式中消除独立项 会收到不同的结果。
答案 0 :(得分：0)
之所以结果不匹配，是因为Statsmodels根据高多重共线性对预测变量进行了预选择。
通过回归的描述性摘要并确定缺少的变量，可以得到完全相同的结果：
deletex = [
'make_alfa-romero',
'fuel_system_1bbl',
'engine_type_dohc',
'num_of_doors_four'
]
df_num.drop( deletex, axis = 1, inplace = True)
df_num = df_num[df_num.columns].apply(pd.to_numeric, errors='coerce', axis = 1)
X = df_num.drop('price', axis = 1)
y = df_num.price.values
Xc = sm.add_constant(X) # Adds a constant to the model
random.seed(1234)
linear_regression = sm.OLS(y, Xc)
linear_regression.fit().summary()
打印结果：
OLS Regression Results
==============================================================================
Dep. Variable: y R-squared: 0.894
Model: OLS Adj. R-squared: 0.868
Method: Least Squares F-statistic: 35.54
Date: Thu, 21 Feb 2019 Prob (F-statistic): 5.24e-62
Time: 18:16:08 Log-Likelihood: -1899.7
No. Observations: 205 AIC: 3879.
Df Residuals: 165 BIC: 4012.
Df Model: 39
Covariance Type: nonrobust
======================================================================================
coef std err t P>|t| [0.025 0.975]
--------------------------------------------------------------------------------------
const 1.592e+04 1.21e+04 1.320 0.189 -7898.396 3.97e+04
bore 3993.4308 1373.487 2.908 0.004 1281.556 6705.306
compression_ratio -1200.5665 460.681 -2.606 0.010 -2110.156 -290.977
height -80.7141 146.219 -0.552 0.582 -369.417 207.988
peak_rpm -0.5903 0.790 -0.747 0.456 -2.150 0.970
make_audi 6519.7045 2371.807 2.749 0.007 1836.700 1.12e+04
make_bmw 1.427e+04 2292.551 6.223 0.000 9740.771 1.88e+04
make_chevrolet -571.8236 2860.026 -0.200 0.842 -6218.788 5075.141
make_dodge -1186.3430 2261.240 -0.525 0.601 -5651.039 3278.353
make_honda 2779.6496 2891.626 0.961 0.338 -2929.709 8489.009
make_isuzu 3098.9677 2592.645 1.195 0.234 -2020.069 8218.004
make_jaguar 1.752e+04 2416.313 7.252 0.000 1.28e+04 2.23e+04
make_mazda 306.6568 2134.567 0.144 0.886 -3907.929 4521.243
make_mercedes-benz 1.698e+04 2320.871 7.318 0.000 1.24e+04 2.16e+04
make_mercury 2958.1002 3605.739 0.820 0.413 -4161.236 1.01e+04
make_mitsubishi -1188.8337 2284.697 -0.520 0.604 -5699.844 3322.176
make_nissan -1211.5463 2073.422 -0.584 0.560 -5305.405 2882.312
make_peugot 3057.0217 4255.809 0.718 0.474 -5345.841 1.15e+04
make_plymouth -894.5921 2332.746 -0.383 0.702 -5500.473 3711.289
make_porsche 9558.8747 3688.038 2.592 0.010 2277.044 1.68e+04
make_renault -2124.9722 2847.536 -0.746 0.457 -7747.277 3497.333
make_saab 3490.5333 2319.189 1.505 0.134 -1088.579 8069.645
make_subaru -1.636e+04 4002.796 -4.087 0.000 -2.43e+04 -8456.659
make_toyota -770.9677 1911.754 -0.403 0.687 -4545.623 3003.688
make_volkswagen 406.9179 2219.714 0.183 0.855 -3975.788 4789.623
make_volvo 5433.7129 2397.030 2.267 0.025 700.907 1.02e+04
fuel_system_2bbl 2142.1594 2232.214 0.960 0.339 -2265.226 6549.545
fuel_system_4bbl 464.1109 3999.976 0.116 0.908 -7433.624 8361.846
fuel_system_idi 1.991e+04 6622.812 3.007 0.003 6837.439 3.3e+04
fuel_system_mfi 3716.5201 3936.805 0.944 0.347 -4056.488 1.15e+04
fuel_system_mpfi 3964.1109 2267.538 1.748 0.082 -513.019 8441.241
fuel_system_spdi 3240.0003 2719.925 1.191 0.235 -2130.344 8610.344
fuel_system_spfi 932.1959 4019.476 0.232 0.817 -7004.041 8868.433
engine_type_dohcv -1.208e+04 4205.826 -2.872 0.005 -2.04e+04 -3773.504
engine_type_l -4833.9860 3763.812 -1.284 0.201 -1.23e+04 2597.456
engine_type_ohc -4038.8848 1213.598 -3.328 0.001 -6435.067 -1642.702
engine_type_ohcf 9618.9281 3504.600 2.745 0.007 2699.286 1.65e+04
engine_type_ohcv 3051.7629 1445.185 2.112 0.036 198.323 5905.203
engine_type_rotor 1403.9928 3217.402 0.436 0.663 -4948.593 7756.579
num_of_doors_two -419.9640 521.754 -0.805 0.422 -1450.139 610.211
==============================================================================
Omnibus: 65.777 Durbin-Watson: 1.217
Prob(Omnibus): 0.000 Jarque-Bera (JB): 399.594
Skew: 1.059 Prob(JB): 1.70e-87
Kurtosis: 9.504 Cond. No. 3.26e+05
==============================================================================
Warnings:
[1] Standard Errors assume that the covariance matrix of the errors is correctly specified.
[2] The condition number is large, 3.26e+05. This might indicate that there are
strong multicollinearity or other numerical problems.
结果完全等于用Statsmodels进行的第一次呼叫：
random.seed(1234)
lm_python = smf.ols('price ~ make + fuel_system + engine_type + num_of_doors + bore + compression_ratio + height + peak_rpm + 1', data = df)
lm_python.fit().summary()
OLS Regression Results
==============================================================================
Dep. Variable: price R-squared: 0.894
Model: OLS Adj. R-squared: 0.868
Method: Least Squares F-statistic: 35.54
Date: Thu, 21 Feb 2019 Prob (F-statistic): 5.24e-62
Time: 18:17:37 Log-Likelihood: -1899.7
No. Observations: 205 AIC: 3879.
Df Residuals: 165 BIC: 4012.
Df Model: 39
Covariance Type: nonrobust
=========================================================================================
coef std err t P>|t| [0.025 0.975]
-----------------------------------------------------------------------------------------
Intercept 1.592e+04 1.21e+04 1.320 0.189 -7898.396 3.97e+04
make[T.audi] 6519.7045 2371.807 2.749 0.007 1836.700 1.12e+04
make[T.bmw] 1.427e+04 2292.551 6.223 0.000 9740.771 1.88e+04
make[T.chevrolet] -571.8236 2860.026 -0.200 0.842 -6218.788 5075.141
make[T.dodge] -1186.3430 2261.240 -0.525 0.601 -5651.039 3278.353
make[T.honda] 2779.6496 2891.626 0.961 0.338 -2929.709 8489.009
make[T.isuzu] 3098.9677 2592.645 1.195 0.234 -2020.069 8218.004
make[T.jaguar] 1.752e+04 2416.313 7.252 0.000 1.28e+04 2.23e+04
make[T.mazda] 306.6568 2134.567 0.144 0.886 -3907.929 4521.243
make[T.mercedes-benz] 1.698e+04 2320.871 7.318 0.000 1.24e+04 2.16e+04
make[T.mercury] 2958.1002 3605.739 0.820 0.413 -4161.236 1.01e+04
make[T.mitsubishi] -1188.8337 2284.697 -0.520 0.604 -5699.844 3322.176
make[T.nissan] -1211.5463 2073.422 -0.584 0.560 -5305.405 2882.312
make[T.peugot] 3057.0217 4255.809 0.718 0.474 -5345.841 1.15e+04
make[T.plymouth] -894.5921 2332.746 -0.383 0.702 -5500.473 3711.289
make[T.porsche] 9558.8747 3688.038 2.592 0.010 2277.044 1.68e+04
make[T.renault] -2124.9722 2847.536 -0.746 0.457 -7747.277 3497.333
make[T.saab] 3490.5333 2319.189 1.505 0.134 -1088.579 8069.645
make[T.subaru] -1.636e+04 4002.796 -4.087 0.000 -2.43e+04 -8456.659
make[T.toyota] -770.9677 1911.754 -0.403 0.687 -4545.623 3003.688
make[T.volkswagen] 406.9179 2219.714 0.183 0.855 -3975.788 4789.623
make[T.volvo] 5433.7129 2397.030 2.267 0.025 700.907 1.02e+04
fuel_system[T.2bbl] 2142.1594 2232.214 0.960 0.339 -2265.226 6549.545
fuel_system[T.4bbl] 464.1109 3999.976 0.116 0.908 -7433.624 8361.846
fuel_system[T.idi] 1.991e+04 6622.812 3.007 0.003 6837.439 3.3e+04
fuel_system[T.mfi] 3716.5201 3936.805 0.944 0.347 -4056.488 1.15e+04
fuel_system[T.mpfi] 3964.1109 2267.538 1.748 0.082 -513.019 8441.241
fuel_system[T.spdi] 3240.0003 2719.925 1.191 0.235 -2130.344 8610.344
fuel_system[T.spfi] 932.1959 4019.476 0.232 0.817 -7004.041 8868.433
engine_type[T.dohcv] -1.208e+04 4205.826 -2.872 0.005 -2.04e+04 -3773.504
engine_type[T.l] -4833.9860 3763.812 -1.284 0.201 -1.23e+04 2597.456
engine_type[T.ohc] -4038.8848 1213.598 -3.328 0.001 -6435.067 -1642.702
engine_type[T.ohcf] 9618.9281 3504.600 2.745 0.007 2699.286 1.65e+04
engine_type[T.ohcv] 3051.7629 1445.185 2.112 0.036 198.323 5905.203
engine_type[T.rotor] 1403.9928 3217.402 0.436 0.663 -4948.593 7756.579
num_of_doors[T.two] -419.9640 521.754 -0.805 0.422 -1450.139 610.211
bore 3993.4308 1373.487 2.908 0.004 1281.556 6705.306
compression_ratio -1200.5665 460.681 -2.606 0.010 -2110.156 -290.977
height -80.7141 146.219 -0.552 0.582 -369.417 207.988
peak_rpm -0.5903 0.790 -0.747 0.456 -2.150 0.970
==============================================================================
Omnibus: 65.777 Durbin-Watson: 1.217
Prob(Omnibus): 0.000 Jarque-Bera (JB): 399.594
Skew: 1.059 Prob(JB): 1.70e-87
Kurtosis: 9.504 Cond. No. 3.26e+05
==============================================================================
有必要检查预测变量中的对应关系，因为pd.get_dummies会广泛获取所有虚拟变量，而Statsmodels在分类变量选择中应用了N-1级。