Computational Ecology and Software, 2013, 3(3): 44-52
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Estimating significances of differences between slopes: A new methodology and software

Vasco M. N. C. S. Vieira1, Joel Creed2
1MARETEC, Instituto Superior Tenico, Universidade Tenica de Lisboa, Av. Rovisco Pais, 1049-001, Lisboa, Portugal
2Departamento de Ecologia, Instituto de Biologia Roberto Alcantara Gomes, Universidade do Estado do Rio de Janeiro - UERJ, PHLC Sala 220, Rua Sao Francisco Xavier 524, 20.559-900, Rio de Janeiro, RJ, Brazil

Received 16 May 2013;Accepted 20 June 2013;Published online 1 September 2013

Determining the significance of slope differences is a common requirement in studies of self-thinning, ontogeny and sexual dimorphism, among others. This has long been carried out testing for the overlap of the bootstrapped 95% confidence intervals of the slopes. However, the numerical random re-sampling with repetition favours the occurrence of re-combinations yielding largely diverging slopes, widening the confidence intervals and thus increasing the chances of overlooking significant differences. To overcome this problem a permutation test simulating the null hypothesis of no differences between slopes is proposed. This new methodology, when applied both to artificial and factual data, showed an enhanced ability to differentiate slopes.

Keywords slope difference;permutation;bootstrap;overlap;randomization tests.

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