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Annals of Botany 90: 37-42, 2002
© 2002 Annals of Botany Company

Avoiding Bias in Calculations of Relative Growth Rate

WILLIAM A. HOFFMANN*,1 and HENDRIK POORTER2

1 Departamento de Engenharia Florestal, Universidade de Brasília, Caixa Postal 04357, Brasília, DF 70919-970, Brazil and 2 Plant Ecophysiology, Utrecht University, PO Box 800·84, 3508 TB Utrecht, The Netherlands

* For correspondence. Fax 00 55 61 347 5458, e-mail hoffmann{at}unb.br

Received: 3 December 2001; Returned for revision: 22 February 2002; Accepted: 14 March 2002

In classical growth analysis, relative growth rate (RGR) is calculated as RGR = (ln W2 – ln W1)/(t2 – t1), where W1 and W2 are plant dry weights at times t1 and t2. Since RGR is usually calculated using destructive harvests of several individuals, an obvious approach is to substitute W1 and W2 with sample means 1 and 2. Here we demonstrate that this approach yields a biased estimate of RGR whenever the variance of the natural logarithm-transformed plant weight changes through time. This bias increases with an increase in the variance in RGR, in the length of the interval between harvests, or in sample size. The bias can be avoided by using the formula

, where

and

are the means of the natural logarithm-transformed plant weights.

Key words: Relative growth rate, growth analysis, methodology, bias.


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