Confusion Matrixs for Binary classifier

Question

I am new to modeling, and I am practicing building a logistic regression model. I would like to create a confusion matrix, but my code doesn't seem to work.

Here is the code for the model (which works):

from sklearn.model_selection import train_test_split
from sklearn.linear_model import LogisticRegression

#train, test = train_test_split(hp[age], test_size=0.3)

#from sklearn import preprocessing
X = hp['age'].values.reshape((32561,1))
#X = hp[['age','hours-per-week']].values
y = hp['evalinvest'].values

LogReg = LogisticRegression()
LogReg.fit(X,y)
print(LogReg.score(X,y))

0.916710174749

Here is where I am having diffculty:

# Confusion Matrix
import numpy as np
from sklearn.metrics import *
CM = confusion_matrix(X,y)
print ("\n\nConfusion matrix:\n", CM)

---

It runs and outputs results, but I don't feel like it is correct.

Confusion matrix:

 [[  0   0   0 ...,   0   0   0]
 [  0   0   0 ...,   0   0   0]
 [385  10   0 ...,   0   0   0]
 ..., 
 [  1   0   0 ...,   0   0   0]
 [  3   0   0 ...,   0   0   0]
 [ 33  10   0 ...,   0   0   0]]

Then, when I run the following code, it doesn't work:

tn, fp, fn, tp = CM.ravel()
print ("\nTP, TN, FP, FN:", tp, ",", tn, ",", fp, ",", fn)

error:

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-68-dca3ebbdc69a> in <module>()
----> 1 tn, fp, fn, tp = CM.ravel()
      2 print ("\nTP, TN, FP, FN:", tp, ",", tn, ",", fp, ",", fn)

ValueError: too many values to unpack (expected 4)

Answer 1

I think you are confused on many levels here.

• Logistic regression is a classifier. It gives you the probability that given an $x_{i}$, the probability it belongs to a class or classes.
• So you will be having your evalinvest as classes, not continuous values.
• When you call predict method on top of trained LogisticRegression model, it predicts class for each $x_{i}$ in your test set. So you expect a single array of class labels they belong to.
• sklearn.metrics.confusion_matrix function takes in the original class values to the ones that are predicted by the model you had trained and returns what $x_{i}$ has been classified into what class. So it returns an n(c) x n(c) array where n(c) is the number of classes. So the diagonal of the matrix indicates the number of elements of class i being classified as class i. And an ideal model should be expected to get this numbers good. The mistake which you had made you sent in your X (train) matrix into it and y (labels) into it, which is wrong. You are supposed to send in y_true(true labels) and y_pred (predicted labels from the model). Check the documentation for more.
• Ideally the function should have thrown an error. But unfortunately you had only one row in your X and one in y and they both are of same size and it passed assert it had to make.
• And np.ravel converts an multi-dimensional array into 1D array. So if you doing a binary classification, you would be having a 2x2 matrix, whose flattened array would have been four elements and the assignment would have worked. But the ravelling you had made releases $n(c)^2$ elements. So there you go ValueError. See the docs

I hope my other answer on confusion matrices may clear things a little more.

Hope it clears some mistakes you are making.