Derive 4PL asymptote intervals from a user-supplied response range

Given the lower and upper bounds of where the asymptotes can sit, returns the disjoint intervals used by make_4pl_formula()'s inv_logit reparam. low is mapped to (lower + pad, midpoint - gap/2), up to (midpoint + gap/2, upper - pad). The gap kills label-switching by ensuring up > low always; the pad keeps the asymptotes off the exact boundaries.

Usage

compute_4pl_bounds(lower = 0, upper = 1, pad = 0.001, gap = 0.002)

Arguments

lower, upper

Numeric scalars. The response-scale range that the asymptotes can occupy (0 and 1 for proportion data; 0.85 and 1 for PSII-like sublethal data, etc.).

pad

Absolute padding from lower and upper. Default 0.001.

gap

Absolute gap between the low and up intervals. Default 0.002.

Value

Named list with low_min, low_max, low_w, up_min, up_max, up_w, midpoint.