2451机器学习简单数值题short

Rare Category Survival Caused by Held-Out Rows

题目

A categorical preprocessor keeps a level only if it appears at least 5 times. In train alone, level Z appears 4 times. After the preprocessor is wrongly fit on the full sample, level Z appears to have frequency 7 and is kept. How many held-out Z rows caused the leak?

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