However, the results from this design require careful interpretation. The design does not result in an unbiased representation of all rivers and streams in agricultural settings. For agricultural land use, the focus was limited to the most important agricultural settings within the 20 study areas. Thus, some agricultural activities and related pesticide use that may be very important in a particular part of the nation may not be included. Another possibility is that use of a particular pesticide in the watersheds sampled may have been much greater than average, so that extrapolation to other areas would result in an overestimate of the occurrence of that pesticide.
Similar biases are possible for urban areas as well.
Also see Explanation of bed sediment and tissue data. In some cases,samples were collected at a nearby alternate location.