selfStart.formula {nls} | R Documentation |
A method for the generic function `selfStart' for formula objects.
selfStart(model, initial, parameters, template)
model |
a nonlinear formula object of the form ~expression . |
initial |
a function object, taking three arguments: mCall ,
data , and LHS , representing, respectively, a matched
call to the function model , a data frame in
which to interpret the variables in mCall , and the expression
from the left-hand side of the model formula in the call to nls .
This function should return initial values for the parameters in
model . |
parameters |
a character vector specifying the terms on the right
hand side of model for which initial estimates should be
calculated. Passed as the namevec argument to the deriv
function. |
template |
an optional prototype for the calling sequence of the
returned object, passed as the function.arg argument to the
deriv function. By default, a template is generated with the
covariates in model coming first and the parameters in
model coming last in the calling sequence. |
a function object of class selfStart
, obtained by applying
deriv
to the right hand side of the model
formula. An
initial
attribute (defined by the initial
argument) is
added to the function to calculate starting estimates for the
parameters in the model automatically.
Jose Pinheiro and Douglas Bates
## self-starting logistic model SSlogis <- selfStart(~ Asym/(1 + exp((xmid - x)/scal)), function(mCall, data, LHS) { xy <- sortedXyData(mCall[["x"]], LHS, data) if(nrow(xy) < 4) { stop("Too few distinct x values to fit a logistic") } z <- xy[["y"]] if (min(z) <= 0) { z <- z + 0.05 * max(z) } # avoid zeroes z <- z/(1.05 * max(z)) # scale to within unit height xy[["z"]] <- log(z/(1 - z)) # logit transformation aux <- coef(lm(x ~ z, xy)) parameters(xy) <- list(xmid = aux[1], scal = aux[2]) pars <- as.vector(coef(nls(y ~ 1/(1 + exp((xmid - x)/scal)), data = xy, algorithm = "plinear"))) value <- c(pars[3], pars[1], pars[2]) names(value) <- mCall[c("Asym", "xmid", "scal")] value }, c("Asym", "xmid", "scal"))