πŸ“ƒ Solution for Exercise M1.03ΒΆ

The goal of this exercise is to compare the performance of our classifier in the previous notebook (roughly 81% accuracy with LogisticRegression) to some simple baseline classifiers. The simplest baseline classifier is one that always predicts the same class, irrespective of the input data.

  • What would be the score of a model that always predicts ' >50K'?

  • What would be the score of a model that always predicts ' <=50K'?

  • Is 81% or 82% accuracy a good score for this problem?

Use a DummyClassifier and do a train-test split to evaluate its accuracy on the test set. This link shows a few examples of how to evaluate the generalization performance of these baseline models.

import pandas as pd

adult_census = pd.read_csv("../datasets/adult-census.csv")

We will first split our dataset to have the target separated from the data used to train our predictive model.

target_name = "class"
target = adult_census[target_name]
data = adult_census.drop(columns=target_name)

We start by selecting only the numerical columns as seen in the previous notebook.

numerical_columns = [
    "age", "capital-gain", "capital-loss", "hours-per-week"]

data_numeric = data[numerical_columns]

Split the data and target into a train and test set.

from sklearn.model_selection import train_test_split

# solution
data_numeric_train, data_numeric_test, target_train, target_test = \
    train_test_split(data_numeric, target, random_state=42)

Use a DummyClassifier such that the resulting classifier will always predict the class ' >50K'. What is the accuracy score on the test set? Repeat the experiment by always predicting the class ' <=50K'.

Hint: you can set the strategy parameter of the DummyClassifier to achieve the desired behavior.

from sklearn.dummy import DummyClassifier

# solution
class_to_predict = " >50K"
high_revenue_clf = DummyClassifier(strategy="constant",
                                   constant=class_to_predict)
high_revenue_clf.fit(data_numeric_train, target_train)
score = high_revenue_clf.score(data_numeric_test, target_test)
print(f"Accuracy of a model predicting only high revenue: {score:.3f}")
Accuracy of a model predicting only high revenue: 0.234

We clearly see that the score is below 0.5 which might be surprising at first. We will now check the generalization performance of a model which always predict the low revenue class, i.e. " <=50K".

class_to_predict = " <=50K"
low_revenue_clf = DummyClassifier(strategy="constant",
                                  constant=class_to_predict)
low_revenue_clf.fit(data_numeric_train, target_train)
score = low_revenue_clf.score(data_numeric_test, target_test)
print(f"Accuracy of a model predicting only low revenue: {score:.3f}")
Accuracy of a model predicting only low revenue: 0.766

We observe that this model has an accuracy higher than 0.5. This is due to the fact that we have 3/4 of the target belonging to low-revenue class.

Therefore, any predictive model giving results below this dummy classifier will not be helpful.

adult_census["class"].value_counts()
 <=50K    37155
 >50K     11687
Name: class, dtype: int64
(target == " <=50K").mean()
0.7607182343065395

In practice, we could have the strategy "most_frequent" to predict the class that appears the most in the training target.

most_freq_revenue_clf = DummyClassifier(strategy="most_frequent")
most_freq_revenue_clf.fit(data_numeric_train, target_train)
score = most_freq_revenue_clf.score(data_numeric_test, target_test)
print(f"Accuracy of a model predicting the most frequent class: {score:.3f}")
Accuracy of a model predicting the most frequent class: 0.766

So the LogisticRegression accuracy (roughly 81%) seems better than the DummyClassifier accuracy (roughly 76%). In a way it is a bit reassuring, using a machine learning model gives you a better performance than always predicting the majority class, i.e. the low income class " <=50K".