Learning curves in ML Python

Introduction

Learning curves help us see how well a model learns over time. They show if the model is improving or stuck.

When you want to check if your model is learning well with more data.

When you want to find out if your model is too simple or too complex.

When you want to decide if adding more training data will help your model.

When you want to compare different models or settings to pick the best one.

Syntax

ML Python

from sklearn.model_selection import learning_curve

train_sizes, train_scores, test_scores = learning_curve(
    estimator, X, y, cv=5, train_sizes=[0.1, 0.3, 0.5, 0.7, 1.0])

estimator is your machine learning model.

X and y are your data and labels.

Examples

Basic example using logistic regression with default settings.

ML Python

from sklearn.linear_model import LogisticRegression
from sklearn.model_selection import learning_curve

model = LogisticRegression()
train_sizes, train_scores, test_scores = learning_curve(model, X, y)

Specifying 3-fold cross-validation and custom training sizes.

ML Python

train_sizes, train_scores, test_scores = learning_curve(
    model, X, y, cv=3, train_sizes=[0.2, 0.4, 0.6, 0.8, 1.0])

Sample Model

This code loads the iris flower data, trains a logistic regression model on different amounts of data, and prints the average training and testing accuracy for each size.

ML Python

from sklearn.datasets import load_iris
from sklearn.linear_model import LogisticRegression
from sklearn.model_selection import learning_curve
import numpy as np

# Load data
X, y = load_iris(return_X_y=True)

# Create model
model = LogisticRegression(max_iter=200)

# Get learning curve data
train_sizes, train_scores, test_scores = learning_curve(
    model, X, y, cv=5, train_sizes=np.linspace(0.1, 1.0, 5))

# Calculate mean scores
train_mean = np.mean(train_scores, axis=1)
test_mean = np.mean(test_scores, axis=1)

# Print results
for size, train_score, test_score in zip(train_sizes, train_mean, test_mean):
    print(f"Training size: {size}, Train score: {train_score:.3f}, Test score: {test_score:.3f}")

Important Notes

Learning curves show training and testing scores as the training size grows.

If training and testing scores are close and high, the model is good.

If training score is high but testing score is low, the model may be overfitting.

Summary

Learning curves help check how well a model learns with more data.

They show if a model is too simple, too complex, or just right.

Use them to decide if you need more data or a different model.

Practice

1. What does a learning curve primarily show in machine learning?

easy

A. The hardware used for training the model

B. How model performance changes with more training data

C. The number of features in the dataset

D. The exact time a model takes to train

Solution

1. Step 1: Understand the purpose of learning curves

   Learning curves plot model performance (like accuracy or error) against the amount of training data.

2. Step 2: Identify what is shown on the curve

   The curve shows how well the model learns as it sees more data, helping to spot underfitting or overfitting.

3. Final Answer:

   How model performance changes with more training data -> Option B

4. Quick Check:

   Learning curve = model performance vs data size [OK]

Hint: Learning curves show performance vs data size [OK]

Common Mistakes:

• Confusing learning curves with training time plots
• Thinking learning curves show feature counts
• Assuming hardware affects learning curves directly

2. Which of the following is the correct way to plot a learning curve in Python using scikit-learn?

easy

A. import sklearn.plot as sp; sp.learning_curve(model, X, y)

B. from sklearn.learning import curve_plot; curve_plot(model, X, y)

C. from sklearn.model_selection import learning_curve; train_sizes, train_scores, test_scores = learning_curve(model, X, y)

D. from sklearn.model_selection import train_curve; train_curve(model, X, y)

Solution

1. Step 1: Recall the correct scikit-learn function for learning curves

   The function is learning_curve from sklearn.model_selection.

2. Step 2: Check the import and usage syntax

   from sklearn.model_selection import learning_curve; train_sizes, train_scores, test_scores = learning_curve(model, X, y) correctly imports learning_curve and calls it with model, features X, and labels y.

3. Final Answer:

   from sklearn.model_selection import learning_curve; train_sizes, train_scores, test_scores = learning_curve(model, X, y) -> Option C

4. Quick Check:

   Correct import and call = from sklearn.model_selection import learning_curve; train_sizes, train_scores, test_scores = learning_curve(model, X, y) [OK]

Hint: Use sklearn.model_selection.learning_curve to plot learning curves [OK]

Common Mistakes:

• Using wrong module or function names
• Trying to import non-existent functions
• Confusing learning_curve with train_test_split

3. Given this code snippet, what will be the shape of train_sizes if X has 100 samples?

from sklearn.model_selection import learning_curve
train_sizes, train_scores, test_scores = learning_curve(model, X, y, train_sizes=[0.1, 0.5, 1.0])

medium

A. (100,)

B. (1,)

C. (3, 100)

D. (3,)

Solution

1. Step 1: Understand train_sizes parameter

   train_sizes is given as a list with 3 values: 0.1, 0.5, and 1.0, representing fractions of the training data.

2. Step 2: Determine output shape of train_sizes

   The learning_curve function returns train_sizes as an array with length equal to the number of training sizes specified, so shape is (3,).

3. Final Answer:

   (3,) -> Option D

4. Quick Check:

   train_sizes length = number of fractions given [OK]

Hint: train_sizes shape matches number of training fractions [OK]

Common Mistakes:

• Assuming train_sizes matches number of samples
• Confusing train_sizes with train_scores shape
• Thinking train_sizes is always (100,)

4. You plotted a learning curve but see both training and validation errors are high and close. What is the likely issue?

medium

A. The model is underfitting the data

B. The model is overfitting the data

C. There is a data leakage problem

D. The training data is too large

Solution

1. Step 1: Analyze error levels on learning curve

   High training error means the model cannot fit the training data well.

2. Step 2: Compare training and validation errors

   Both errors being high and close indicates underfitting, where the model is too simple.

3. Final Answer:

   The model is underfitting the data -> Option A

4. Quick Check:

   High train & val error = underfitting [OK]

Hint: High train and val errors mean underfitting [OK]

Common Mistakes:

• Confusing underfitting with overfitting
• Assuming data leakage causes high errors
• Thinking large data size causes this pattern

5. You have a learning curve where training error is very low but validation error is high and does not improve with more data. What is the best next step?

hard

A. Try a simpler model to reduce overfitting

B. Collect more training data to improve validation

C. Increase model complexity to fit training data better

D. Stop training because the model is perfect

Solution

1. Step 1: Interpret the learning curve pattern

   Low training error but high validation error means the model fits training data too well but fails on new data, indicating overfitting.

2. Step 2: Decide the best action to fix overfitting

   Using a simpler model reduces complexity and helps generalize better, improving validation error.

3. Final Answer:

   Try a simpler model to reduce overfitting -> Option A

4. Quick Check:

   Low train error + high val error = overfitting; simplify model [OK]

Hint: Low train error + high val error means overfitting; simplify model [OK]

Common Mistakes:

• Collecting more data won't fix overfitting alone
• Increasing complexity worsens overfitting
• Assuming model is perfect with low training error