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.
USAGE:
SSlogis(input, Asym, xmid, scal)
REQUIRED ARGUMENTS:
input
a numeric vector of values at which to evaluate the model.
Asym
a numeric parameter representing the asymptote.
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.
scal
a numeric scale parameter on the
input axis.
VALUE:
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.