How were the NLA sites selected?
To select lakes for sampling, EPA used a statistical sampling approach incorporating state-of-the-art survey design techniques developed by EPA’s research program. These techniques, based on the same statistical principles as election polls, make it possible to sample a relatively small number of sites and make unbiased estimates about conditions across the entire population under study with statistical confidence. The greater the number of sites sampled, the more confidence in the results and the smaller the margin of error. The number of sites included in the survey allows EPA to determine the percentage of lakes nationwide and within predetermined ecoregions that exceed a threshold of concern with 95% confidence. EPA used the National Hydrography Dataset (NHD) to compile a list of target lakes and their locations. (The NHD is a multi-layered series of digital maps that reveal key attributes of the nation’s waters, such as topography, area, flow and location.). A total of 1,208 lakes
