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Probability Sampling
Description: The hallmark of EPA's Environmental Monitoring and Assessment Program (EMAP) is the selection of water bodies (or sites within water bodies) using a probability-based sampling design. Sites within a region are randomly selected for sampling and the results from the random sample can then be applied to the entire region with a known level of uncertainty.
Simple example: You are sampling a stream site and have defined 10 transects across the stream for taking channel measurements. You also want to sample benthic invertebrates but do not want to sample from every transect. To randomly select the sample sites, you could use a random numbers table to select 1 of the 10 transects.
MAIA example: Using samples from 562 stream sites, the condition of the invertebrate assemblage in the MAIA region was evaluated. To develop the multimetric index for invertebrates, some additional sites were selected based on judgment sampling. These sampling sites represented the extremes of site condition and included reference (or least impaired) and test (or degraded) sites.
Figure
Figure. Probability sampling sites (yellow), reference sites (blue and green) and test (sites) for benthic invertebrates. Probability sites were randomly located within the region.
How the method works: EMAP's probability sampling design is called a Generalized Random Tessellation Stratified (GRTS) spatially-balanced survey design. The method differs from a simple random sample in that sampling locations are more evenly spread throughout the region and the clumping pattern typical of simple random sampling is replaced by a more even distribution of sampling locations. Sites are still randomly selected in a GRTS design by using a hierarchical organizational scheme that orders all possible sites in the sampling frame in such a way that a simple selection process will yield a spatially-balanced random design no matter how many sites are chosen.