Application of repeated measures designs in environmental impact and monitoring studies

Abstract

Traditional environmental studies have employed sampling at different times, but based on re-randomized 'replicate' samples taken at each time. For example, in a 4 year monitoring study of near-shore marine benthic communities there might be three box cores collected annually at each of three depths along each of three transects. Repeated measures designs, long used in medicine and the social sciences, are based on resampling replicates (e.g. sites) at a series of times. In such designs spatial sampling variability is not used for tests of environmental impact. Error for such tests is based on variability of time trends among similar sites (similar with respect to impact). For example in a tropical oil spill study five oiled and five unoiled coral reefs were studied over 5 years. Error for tests of oil impact was based on variability among reefs (within degree-of-oiling category) in the year-to-year trends of biological response variables. It was not based on variability among field samples within reefs at given times.The two approaches (univariate and multivariate) to repeated measures analysis of variance are described. The pros and cons of each are discussed, as are the assumptions and consequences of their violations. Emphasis is especially placed on the adequacy of error degrees of freedom in the two approaches, and some exploration of power to detect impact is presented. Examples of application of repeated measures designs to various impact and monitoring studies are presented and discussed, including (i) interpretation of significant effects; (ii) decomposition of effects by contrasts (e.g. before vs after impact); and (iii) modelling time trends by polynomial and cosine functions.

Efeito de herbicidas sobre a comunidade de artrópodes do solo do feijoeiro cultivado em sistema de plantio direto e convencional

“…Uma vez que a amostragem dos artrópodes foi realizada no mesmo campo várias vezes, a análise de variância por medida repetida é recomendada para evitar o probl ema de pse udo -re pli cação no tempo (Hurlbert, 1984;Stewart-Oater et al, 1986;Green, 1993;Paine, 1996) . Essas análises foram feitas usando o procedimento ANOVA do SAS com a especificação PROFILE, como sugerido por von Ende (1993).…”

Effects of anthropogenic and natural disturbance on a recently settled continental shelf flatfish

“…Recently settled Limanda ferruginea and major prey-taxa abundance data were analyzed using a univariate 'repeated measures' ANOVA because individual boxes nested within treatments were re-sampled several times over the course of 1 yr (Green 1993, Underwood 1993. One advantage of this design is that tests of impact-related hypotheses are not based on the error derived from random sub-sampling within sites (in this case, treatment boxes; 'pseudoreplication', see Hurlbert 1984).…”

“…Both components (nursery habitat, fishing grounds) are overlain on a seasonal, storm-driven system punctuated by rare, high magnitude events (hurricanes, winter storms; Butman 1987). The present study used a spatially replicated BACI (before-after-control-impact) experimental design (Underwood 1991, Green 1993 to investigate the effects of mobile, commercial scallop dredge gear on New York Bight young-of-the-year (YOY) yellowtail flounder Limanda ferruginea and its benthic nursery habitat. The following 4 null hypotheses concerning the spatial and temporal effects of a single dredging event were considered: specifically, that there is no significant difference from before to after dredging in (H 1 ) YOY Limanda ferruginea abundance, (H 2 ) L. ferruginea size structure, (H 3 ) benthic habitat structure, and (H 4 ) benthic prey abundance.…”

Journal of Range Management, Volume 53, Number 6 (November 2000)

“…Under anaerobic conditions, microbial processes help produce reduced forms of elements such as carbon, nitrogen, and sulphur that can be released into the atmosphere (Hussey et al 1985, Schlesinger 1991, Hill 1996, Schnabel et al 1996. Riparian areas can serve as a sink for phosphorus-laden sediments, further upgrading water quality (Green andKauffman 1989, Corley et al 1999). The presence of herbaceous vegetation improves water quality by enhancing sediment deposition.…”