Assessing effects of unreplicated perturbations: No simple solutions
- University of California, Santa Barbara (United States)
The authors address the task of determining the effects, on mean population density or other parameters, of an unreplicated perturbation, such as arises in environmental assessments and some ecosystem-level experiments. The authors context is the Before-After-Control-Impact-Pairs design (BACIP): on several dates Before and After the perturbation, samples are collected simultaneously at both the Impact site and a nearby 'Control'. One approach is to test whether the mean of the Impact-Control difference has changed from Before to After the perturbation. If a conventional test is used, checks of its assumptions are an important and messy part of the analysis, since BACIP data do not necessarily satisfy them. It has been suggested that these checks are not needed for randomization tests, because they are insensitive to some of these assumptions and can be adjusted to allow for others. A major aim of this paper is to refute this suggestion: there is no panacea for the difficult and messy technical problems in the analysis of data from assessments or unreplicated experiments. They compare the randomization t test with the standard t test and the modified (Welch-Satterthwaite-Aspin) t test, which allows for unequal variances.
- DOE Contract Number:
- FG03-89ER60885
- OSTI ID:
- 7177222
- Journal Information:
- Ecology; (United States), Vol. 73:4; ISSN 0012-9658
- Country of Publication:
- United States
- Language:
- English
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