Peterson, Spencer A. and N.S. Urquhart. 2000. Estimating trophic state proportions of a regional lake population: are larger samples always better? Environmental Monitoring and Assessment 62:71-89. NHEERL-COR-2207J
During the summers of 1991-1994, the Environmental Monitoring and Assessment Program (EMAP) sampled 344 lakes throughout the northeastern United States using a proportional stratified sampling design based on lake size. Approximately one-quarter of the 344 lakes were sampled each year (4 years) for total phosphorus to determine the proportion (and associated 95% confidence intervals) of the northeast lake population # 1 ha (11,076 ± 1,699 lakes) that was in oligotrophic, mesotrophic, eutrophic. Or heupereutropic (4 classes) condition according to the total phosphorus criteria of the North American Lake Management Society. Estimates for the second, third, and fourth yr were developed as cumulative of the previous yr samples and the current yr samples for the northeast as a whole and for each of its three ecoregions (4 regions). New confidence intervals were computed for each cumulative yr condition estimate., This produced a total (4 years x 4 classes x 4 regions) of 64 cumulative yr tropic condition estimates. Confidence intervals for 21% of these estimates did not shorten with increased sample size. This phenomena raised questions about the accuracy of estimates based on cumulative sampling procedures. We explain why and how the phenomenon comes about with both straight random and proportional random sampling. Further, we present an example of the effects this phenomenon has on lake tropic state condition estimates in the northeastern United States.