# Logistic Regression: Scikit Learn vs glmnet

I am trying to duplicate the results from sklearn logistic regression library using glmnet package in R.

From the sklearn logistic regression documentation, it is trying to minimize the cost function under l2 penalty

From the vignettes of glmnet, its implementation minimizes a slightly different cost function

With some tweak in the second equation, and by setting $\alpha=0$,

which differs from sklearn cost function only by a factor of $\lambda$ if set $\frac1{N\lambda}=C$, so I was expecting the same coefficient estimation from the two packages. But they are different. I am using the dataset from UCLA idre tutorial, predicting admit based on gre, gpa and rank. There are 400 observations, so with $C=1$, $\lambda = 0.0025$.

#python sklearn
y, X = dmatrices('admit ~ gre + gpa + C(rank)', df, return_type = 'dataframe')
>  Intercept  C(rank)[T.2]  C(rank)[T.3]  C(rank)[T.4]  gre   gpa
0          1             0             1             0  380  3.61
1          1             0             1             0  660  3.67
2          1             0             0             0  800  4.00
3          1             0             0             1  640  3.19
4          1             0             0             1  520  2.93

model = LogisticRegression(fit_intercept = False, C = 1)
mdl = model.fit(X, y)
model.coef_
> array([[-1.35417783, -0.71628751, -1.26038726, -1.49762706,  0.00169198,
0.13992661]])
# corresponding to predictors [Intercept, rank_2, rank_3, rank_4, gre, gpa]

> # R glmnet
> mylogit <- glmnet(X, y, family = "binomial", alpha = 0)
> coef(mylogit, s = 0.0025)
6 x 1 sparse Matrix of class "dgCMatrix"
1
(Intercept)      -3.984226893
gre               0.002216795
gpa               0.772048342
as.factor(rank)2 -0.530731081
as.factor(rank)3 -1.164306231
as.factor(rank)4 -1.354160642


The R output is somehow close to logistic regression without regularization, as can be seen here. Am I missing something or doing something obviously wrong?

Update: I also tried to use LiblineaR package in R to conduct the same process, and yet got another different set of estimates (liblinear is also the solver in sklearn):

> fit = LiblineaR(X, y, type = 0, cost = 1)
> print(fit)
$TypeDetail [1] "L2-regularized logistic regression primal (L2R_LR)"$Type
[1] 0
\$W
gre          gpa as.factor(rank)2 as.factor(rank)3 as.factor(rank)4         Bias
[1,] 0.00113215 7.321421e-06     5.354841e-07     1.353818e-06      9.59564e-07 2.395513e-06


Update 2: turning off standardization in glmnet gives:

> mylogit <- glmnet(X, y, family = "binomial", alpha = 0, standardize = F)
> coef(mylogit, s = 0.0025)
6 x 1 sparse Matrix of class "dgCMatrix"
1
(Intercept)      -2.8180677693
gre               0.0034434192
gpa               0.0001882333
as.factor(rank)2  0.0001268816
as.factor(rank)3 -0.0002259491
as.factor(rank)4 -0.0002028832


Dougal’s answer is correct, you regularize the intercept in sklearn but not in R. Make sure you use solver='newton-cg' since default solver ('liblinear') always regularizes the intercept.