| logistic4_fn {drda} | R Documentation |
Evaluate at a particular set of parameters the 4-parameter 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 4-parameter 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), alpha is the value of the
function when x -> -Inf, delta is the (signed) height of the curve,
eta > 0 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 mid-point.
When delta < 0 the curve is monotonically decreasing while it is
monotonically increasing for delta > 0.
The mid-point f(phi; theta) is equal to alpha + delta / 2 while the value
of the function for x -> Inf is alpha + delta.
Numeric vector of the same length of x with the values of the
logistic function.