What sentinel sites can and cannot represent

Sites selected for sensitivity or continuity can provide excellent process evidence without representing a region. This methods note combines primary preferential-sampling research, EPA probability-survey guidance, and climate-observing sources to examine that distinction. A fictional catchment census shows how a sentinel network concentrated in sensitive terrain can yield a site fraction far above the regional proportion. The analysis separates population targets, weighting, purposeful selection, probability sampling, and conditional model-based expansion. It considers hybrid networks, transfer-oriented evaluation, changing representativeness, and protected selection records. No actual regional condition or optimal site list is estimated. The implication is to retain detailed sentinel findings and separately justified regional estimates as complementary products, rather than treating repeated measurements at informative sites as a cure for missing population coverage.