In the prac, what does test_size=0.4 in train_test_split() mean?

Question

BlackTom AI · Accepted Answer

test_size=0.4 specifies the fraction of the entire dataset that will be placed into the test (hold-out) set: 40% for testing and the remaining 60% for training.

1. Relevant knowledge points and concepts
- In scikit-learn's train_test_split(), the parameter test_size can be a float (proportion), an int (absolute number of samples), or None.  
- When given as a float between 0.0 and 1.0, it denotes the proportion of the dataset assigned to the test set. The train set receives the remainder (1 - test_size).

2. Step-by-step reasoning
- Let N be the total number of samples in the dataset.  
- With test_size = 0.4 (a float), the test set size = 0.4 × N.  
- The train set size = N − test_size_count = N − (0.4 × N) = 0.6 × N, i.e., 60% of the data.

3. Calculations with intermediate steps (example)
- If N = 100: test_count = 0.4 × 100 = 40 samples; train_count = 100 − 40 = 60 samples.  
- If N = 250: test_count = 0.4 × 250 = 100 samples; train_count = 250 − 100 = 150 samples.

4. Why the correct answer is correct
- Because train_test_split interprets 0.4 as a proportion for the test set, that leaves 60% (1 − 0.4) for training. Therefore "60% of the data is used for training and 40% is reserved for testing" directly follows from this interpretation.

5. Why the other options are incorrect
- "40% of the data is used for training and 60% for testing" reverses the roles of train and test—incorrect for test_size=0.4.  
- "The data is split into 4 equal folds" describes k-fold cross-validation with k=4, not a single train/test split.  
- "40 samples are randomly selected for the test set" would only be correct if test_size were the integer 40 and N unspecified; here test_size is a float (0.4), so it denotes proportion, not an absolute count.

6. Conclusion
- The correct interpretation is: 60% of the data is used for training and 40% is reserved for testing, which matches the provided answer.

In the prac, what does test_size=0.4 in train_test_split() mean?单项选择题

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