An R function: OLS/Robust scaled mass index

R code

	# Inspired by Peig & Green
	# "New perspectives for estimating body condition from mass/length data:
	# the scaled mass index as an alternative method"
	# Oikos 118: 1883-1891, 2009
	# Author: Chen-Pan Liao
	scaledMassIndex <-
	function(x, y, x.0 = mean(x)) {
	require(smatr)
	require(magrittr)
	require(MASS)
	require(data.table)
	logM.ols <- lm(log(y) ~ log(x))
	logM.rob <- rlm(log(y) ~ log(x), method = "M")
	b.msa.ols <- coef(sma(log(y) ~ log(x)))[2]
	b.msa.rob <- coef(sma(log(y) ~ log(x), robust = T))[2]
	SMI.ols <- y * (x.0 / x) ^ b.msa.ols
	SMI.rob <- y * (x.0 / x) ^ b.msa.rob
	res <- data.frame(SMI.ols, SMI.rob, x, y)
	pred.DT <-
	data.table(x = seq(min(x), max(x), length = 100)) %>%
	.[, y.ols := predict(logM.ols, newdata = .) %>% exp] %>%
	.[, y.rob := predict(logM.rob, newdata = .) %>% exp]
	attr(res, "b.msa") <- c(ols = b.msa.ols, rob = b.msa.rob)
	return(res)
	}

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