Farm-Scale Evaluations of spring-sown genetically modified herbicide-tolerant crops: a statistical assessment

Clark, Suzanne J.; Rothery, Peter; Perry, Joe N.. 2006 Farm-Scale Evaluations of spring-sown genetically modified herbicide-tolerant crops: a statistical assessment. Proceedings of the Royal Society (B), 273 (1583). 237-243.

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Primary results from the Farm Scale Evaluations (FSEs) of spring-sown genetically modified herbicide-tolerant crops were published in 2003. We provide a statistical assessment of the results for count data, addressing issues of sample size (n), efficiency, power, statistical significance, variability and model selection. Treatment effects were consistent between rare and abundant species. Coefficients of variation averaged 73% but varied widely. High variability in vegetation indicators was usually offset by large n and treatment effects, whilst invertebrate indicators often had smaller n and lower variability; overall, achieved power was broadly consistent across indicators. Inferences about treatment effects were robust to model misspecification, justifying the statistical model adopted. As expected, increases in n would improve detectability of effects whilst, for example, halving n would have resulted in a loss of significant results of about the same order. 40% of the 531 published analyses had greater than 80% power to detect a 1.5-fold effect; reducing n by one-third would most likely halve the number of analyses meeting this criterion. Overall, the data collected vindicated the initial statistical power analysis and the planned replication. The FSEs provide a valuable database of variability and estimates of power under various sample size scenarios to aid planning of more efficient future studies

Item Type: Publication - Article
Digital Object Identifier (DOI):
Programmes: CEH Programmes pre-2009 publications > Biodiversity
UKCEH and CEH Sections/Science Areas: _ Ecological Processes & Modelling
Format Availability: Electronic, Print
Additional Keywords: farmland wildlife, count data, treatment effects, variability, sample size, statistical power
NORA Subject Terms: Agriculture and Soil Science
Date made live: 29 Jun 2007 14:17 +0 (UTC)

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