.\" -*- nroff -*- generated from .Rd format
.BG
.FN SSlogis
.TL
Logistic model
.DN
This `selfStart' model evaluates the logistic
function and its gradient.  It has an `initial' attribute that
will evaluate initial estimates of the parameters `Asym',
`xmid', and `scal' for a given set of data.
.CS
SSlogis(input, Asym, xmid, scal)
.RA
.AG input
a numeric vector of values at which to evaluate the model.
.AG Asym
a numeric parameter representing the asymptote.
.AG xmid
a numeric parameter representing the `x' value at the
inflection point of the curve.  The value of `SSlogis' will be
`Asym/2' at `xmid'.
.AG scal
a numeric scale parameter on the `input' axis.
.RT
a numeric vector of the same length as `input'.  It is the value of
the expression `Asym/(1+exp((xmid-input)/scal))'.  If all of
the arguments `Asym', `xmid', and `scal' are
names of objects, as opposed to expressions or explicit numerical
values, the gradient matrix with respect to these names is attached as
an attribute named `gradient'.

.SA
`nls', `selfStart'
.EX
Chick.1 <- ChickWeight[ChickWeight$Chick == 1, ]
SSlogis( Chick.1$Time, 368, 14, 6 )  # response only
Asym <- 368
xmid <- 14
scal <- 6
SSlogis( Chick.1$Time, Asym, xmid, scal ) # response and gradient
.KW models
.WR
