logistic2_fn {drda}  R Documentation 
2parameter logistic function
Description
Evaluate at a particular set of parameters the 2parameter logistic function.
Usage
logistic2_fn(x, theta)
Arguments
x 
numeric vector at which the logistic function is to be evaluated. 
theta 
numeric vector with the four parameters in the form

Details
The 2parameter logistic function f(x; theta)
is defined here as
g(x; theta) = 1 / (1 + exp(eta * (x  phi)))
f(x; theta) = alpha + delta g(x; theta)
where theta = c(alpha, delta, eta, phi)
and eta > 0
. Only eta
and phi
are free to vary (therefore the name) while vector c(alpha, delta)
is
constrained to be either c(0, 1)
(monotonically increasing curve) or
c(1, 1)
(monotonically decreasing curve).
This function allows values other than 0, 1, 1 for alpha
and delta
but
will coerce them to their proper constraints.
Value
Numeric vector of the same length of x
with the values of the
logistic function.