How were the random sites selected?
The 1,392 sites sampled for the WSA were identified using a particular type of random sampling technique called a probability-based sample design, in which every element in the population has a known probability of being selected for sampling. Rules for site selection included weighting to provide balance in the number of stream sites from each of the 1st through 5th order size classes and controlled spatial distribution to ensure that sample sites were distributed across the United States. The WSA random sites were allocated by EPA region and by ecological region, based on the distribution of 1st through 5th order streams within those regions. Within each EPA region, the random sites are more densely distributed where the perennial 1st through 5th order streams are more densely located. Sites are more sparsely distributed where streams are sparse. To pick a random sample, one must first know the location of members of the population of interest. The target population for the WSA was t
