Evaluate at a particular set of parameters the 2-parameter log-logistic function.
loglogistic2_fn(x, theta)Numeric vector of the same length of x with the values of the
log-logistic function.
numeric vector at which the function is to be evaluated.
numeric vector with the four parameters in the form
c(alpha, delta, eta, phi). alpha can only be equal to 0 or 1 while
delta can only be equal to 1 or -1.
The 2-parameter log-logistic function f(x; theta) is defined here as
g(x; theta) = x^eta / (x^eta + phi^eta)
f(x; theta) = alpha + delta g(x; theta)
where x >= 0, theta = c(alpha, delta, eta, phi), eta > 0, and
phi > 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.