logistic4_fn {drda}  R Documentation 
Evaluate at a particular set of parameters the 4parameter logistic function.
logistic4_fn(x, theta)
x 
numeric vector at which the logistic function is to be evaluated. 
theta 
numeric vector with the four parameters in the form

The 4parameter logistic function f(x; theta)
is defined here as
alpha + (beta  alpha) / (1 + exp(eta * (x  phi)))
where theta = c(alpha, beta, eta, phi)
, alpha
is the lower horizontal
asymptote, beta > alpha
is the upper horizontal asymptote, eta
is the
steepness of the curve or growth rate (also known as the Hill coefficient),
and phi
is the value of x
at which the curve is equal to its midpoint.
Numeric vector of the same length of x
with the values of the
logistic function.