A subpopulation is a defined subset of a broader, heterogeneous population. Segmenting users or patients by relevant characteristics helps reveal effects that population averages can hide. section 2.3.1
Conditional probability provides the operational filter. In P(E | F=True, G="millennial"), the target is users exposed to a feature who also belong to the specified demographic subgroup. section 2.3.1
A factual condition narrows the population to cases where an observed fact holds. It differs from a hypothetical intervention: P(I | E="high") describes purchases among players with high engagement. section 8.2.2
Targeting a feature
A website feature may increase engagement on average while reducing engagement for a particular subpopulation. Marketing segmentation therefore targets the feature toward groups for which the observed effect is useful; precision medicine applies the same logic to patient benefit. section 2.3.1
