Four-parameter Logistic Model
Usage
SSfpl(input, A, B, xmid, scal)
Arguments
input
|
a numeric vector of values at which to evaluate the model.
|
A
|
a numeric parameter representing the horizontal asymptote on
the left side (very small values of input ).
|
B
|
a numeric parameter representing the horizontal asymptote on
the right side (very large values of input ).
|
xmid
|
a numeric parameter representing the input value at the
inflection point of the curve. The value of SSfpl will be
midway between A and B at xmid .
|
scal
|
a numeric scale parameter on the input axis.
|
Description
This selfStart
model evaluates the four-parameter logistic
function and its gradient. It has an initial
attribute that
will evaluate initial estimates of the parameters A
, B
,
xmid
, and scal
for a given set of data.Value
a numeric vector of the same length as input
. It is the value of
the expression A+(B-A)/(1+exp((xmid-input)/scal))
. If all of
the arguments A
, B
, 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
.Author(s)
Jose Pinheiro and Douglas BatesSee Also
nls
, selfStart
Examples
library( lme )
data( ChickWeight )
Chick.1 <- ChickWeight[ChickWeight$Chick == 1, ]
SSfpl( Chick.1$Time, 13, 368, 14, 6 ) # response only
A <- 13
B <- 368
xmid <- 14
scal <- 6
SSfpl( Chick.1$Time, A, B, xmid, scal ) # response and gradient