2.1.7.2 Opportunistic spatial design pros and cons

An opportunistic spatial design implies that sites are selected on the basis of the investigator’s judgment about what sites best represent the population to be described.  Randomization is not used in the selection of sites.  Sites selected opportunistically are sometimes referred to as "index" sites.

Tools:

None.

Pros and Cons:

Site selection

Pros:

• Can accommodate special settings required for some field protocols
• Can select sites that are convenient, resulting in lower field cost.
• Useful for initial (exploratory) investigations.

 Cons: 

• Sites may not be representative or representativeness is unknown
• Can lead to complacency about the need to develop defensible sampling designs unless biologists are under such fiscal or personnel constraints that they cannot address the need to be able to draw conclusions for areas outside the sampled areas (see limited universe of inference below).

Statistical Inference (summary/interpretation of results)

Pros:

• Can utilize long-term records available from many existing monitoring programs or other data sources, most of which have been opportunistic programs.

Cons:

• Statistical inferences are limited to the set of convenience-based opportunistic sites (in strictest sense), rather than being applicable to a wider universe of inference. Cannot extrapolate to other sites because there is no logical basis for inference about other sites without other types of information. In the absence of such other types of information, estimates for those other sites will usually be biased.
• Require assumptions and models to make inferences to entire population (when they are needed).
• Cannot determine bias and precision associated with inferences to entire population, given only such convenience-based/opportunistic data.