Web Release Date: April 8,
Sample Representativeness: A Must for Reliable Regional Lake Condition Estimates
and
U.S. EPA National Health and Ecological Effects Research Laboratory, Western Ecology Division, Corvallis, Oregon 97333, Department of Statistics, Oregon State University, Corvallis, Oregon 97331, and Department of Civil and Environmental Engineering, University of Washington, Seattle, Washington 98195
Received for review July 14, 1998
Revised manuscript received December 16, 1998
Accepted February 24, 1999
Abstract:
Reliable environmental resource estimates are essential
to informed regional scale decisions regarding protection,
restoration, and enhancement of natural resources.
Reliable estimates depend on objective and representative
sampling. Probability-based sampling meets these
requirements and provides accuracy estimates (confidence
limits). Non-probability-based (judgment or convenience)
sampling often is biased, thus less reliable (no accuracy
estimates), and potentially misleading. We compare results
from a probability- and a non-probability-based Secchi
transparency sampling of lakes in the northeastern geographic
region of the United States and its three primary
ecoregions. Results from these samplings are compared
on the basis of sample representativeness relative to the
regional lake population and subsequent reliability of lake
condition estimates. Statistically derived sampling
indicates 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. On 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 population based on statistically designed
sampling estimates has a SDT 
4.2 m. Estimate unreliability
of this magnitude can have profound effects on lake
management decisions. Thus, regional extrapolation of non-probability-based sampling results should be avoided.
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