Juvenile mysid shrimp (Mysidopsis bahia) were exposed to up to 32% effluent in a 7-day survival and growth test. The average weight per treatment replicate of surviving organisms was measured.
A data frame with 40 observations on the following 2 variables.
a numeric vector of effluent concentrations (%)
a numeric vector of average dry weights (mg)
The data are analysed in Bruce and Versteeg (1992) using a log-normal dose-response model (using the logarithm with base 10).
At 32% there was complete mortality, and this justifies using a model where a lower asymptote of 0 is assumed.
Bruce, R. D. and Versteeg, D. J. (1992) A statistical procedure for modeling continuous toxicity data, Environ. Toxicol. Chem., 11, 1485–1494.
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M.bahia.m1 <- drm(dryweight~conc, data=M.bahia, fct=LN.3()) ## Variation increasing plot(fitted(M.bahia.m1), residuals(M.bahia.m1)) ## Using transform-both-sides approach M.bahia.m2 <- boxcox(M.bahia.m1, method = "anova") summary(M.bahia.m2) # logarithm transformation ## Variation roughly constant, but still not a great fit plot(fitted(M.bahia.m2), residuals(M.bahia.m2)) ## Visual comparison of fits plot(M.bahia.m1, type="all", broken=TRUE) plot(M.bahia.m2, add=TRUE, type="none", broken=TRUE, lty=2) ED(M.bahia.m2, c(10,20,50), ci="fls") ## A better fit M.bahia.m3 <- boxcox(update(M.bahia.m1, fct = LN.4()), method = "anova") #plot(fitted(M.bahia.m3), residuals(M.bahia.m3)) plot(M.bahia.m3, add=TRUE, type="none", broken=TRUE, lty=3, col=2) ED(M.bahia.m3, c(10,20,50), ci="fls")
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