Stratified Sampling
Sampling more rare cases helps estimate their performance, but an overall population rate still needs the population weights. A worked example separates those two goals.
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Negative Control
A negative control removes the signal a pipeline is supposed to detect. Its expected score depends on the task, and its construction determines which defects it can expose.
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Design Effect
Related items contribute less independent information than their row count suggests. The design effect connects cluster size and correlation to variance and effective sample size.