selfStart {stats} | R Documentation |
Construct self-starting nonlinear models.
selfStart(model, initial, parameters, template)
model |
a function object defining a nonlinear model or
a nonlinear formula object of the form |
initial |
a function object, taking three arguments: |
parameters |
a character vector specifying the terms on the right
hand side of |
template |
an optional prototype for the calling sequence of the
returned object, passed as the |
This function is generic; methods functions can be written to handle specific classes of objects.
a function object of class "selfStart"
, for the formula
method 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
nls
## 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"))
# 'first.order.log.model' is a function object defining a first order
# compartment model
# 'first.order.log.initial' is a function object which calculates initial
# values for the parameters in 'first.order.log.model'
# self-starting first order compartment model
## Not run:
SSfol <- selfStart(first.order.log.model, first.order.log.initial)
## End(Not run)