Standardise using training statistics

Question

Write Python functions that fit feature means and population standard deviations on training data and apply them to another matrix. Define a policy for constant columns.

Short answer

Use training statistics unchanged for the other matrix. Replace a zero standard deviation by one so a constant training column becomes zero without division by zero. A new value in that column remains interpretable relative to the training constant.

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Lessons behind this question

The Supervised Learning Problem and the ML Pipeline
Covariance and Correlation
Populations, Samples, and Sampling Distributions

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