Sample representativeness: A must for reliable regional lake condition estimates

Reliable environmental resource estimates are essential to informed regiona l scale decisions regarding protection, restoration, and enhancement of nat ural resources. Reliable estimates depend on objective and representative s ampling. Probability-based sampling meets these requirements and provides a ccuracy estimates (confidence limits). Non-probability-based (judgment or c onvenience) sampling often is biased, thus less reliable (no accuracy estim ates), and potentially misleading. We compare results from a probability- a nd a non-probability-based Secchi transparency sampling of lakes in the nor theastern geographic region of the United States and its three primary ecor egions. Results from these samplings are compared on the basis of sample re presentativeness relative to the regional lake population and subsequent re liability of lake condition estimates. Statistically derived sampling indic ates the northeast lake population median lake size to be about 9.5 (+/- 2. 3) ha and the Secchi disk transparency (SDT) to be about 2.4 (+/- 0.4) m. O n the basis of judgment sampling estimates, the median SDT for lakes in the same area would be 4.2 m. However, only about 15% of the regional lake pop ulation based on statistically designed sampling estimates has a SDT greate r than or equal to 4.2 m. Estimate unreliability of this magnitude can have profound effects on lake management decisions. Thus, regional extrapolatio n of non-probability-based sampling results should be avoided.