The LTRMP has historically collected repeated samples from ‘judgment sites’--locations that were selected with particular site characteristics in mind. In LTRMP parlance and in historical LTRMP documents, these judgment sites have traditionally been termed “fixed sites.” Unfortunately, this term has implicitly confused locations that were repeatedly sampled with locations that were selected using a nonprobabilistic mechanism (a site selected using a probabilistic mechanism may, of course, be repeatedly sampled). Hence, the LTRMP term "fixed sites" connotes both the selection of sites using a judgment process and repeated sampling at those sites.

Response information from LTRMP nonprobabilistic locations should not be used for reporting status and trend estimates at the reach or population level. This is because the potential biases associated with selecting a nonrandom set of sampling locations are unknown and, consequently, the representativeness of the sampled locations with respect to unsampled locations in the population is also unknown. Further discussion of this topic is provided by Olsen et al. (1999) and Edwards (1998).

The statements above do not preclude modeling data from nonrandom locations (where, by ‘modeling,’ we mean deriving inferences based on assumptions other than those associated with the sampling design; see Statistical Models and LTRMP Data). Doing so, however, requires that the authority of any model inferences rests on the reasonableness of the model rather, as is often the case with status estimates, on the sampling design.

References

Edwards, D. 1998. Issues and themes for natural resources trend and change detection. Ecological Applications 8:323–325.

Olsen, A. R., J. Sedransk, C. Gotway, W. Liggett, S. Rathbun, K. Reckhow, L. Young, and D. Edwards. 1999. Statistical issues for monitoring ecological and natural resources in the United States. Environmental Monitoring and Assessment 54:1–45.

Contact: Questions or comments may be directed to Brian Gray, LTRMP statistician, Upper Midwest Environmental Sciences Center, La Crosse, Wisconsin, at brgray@usgs.gov.