d.4pl {desiR}

Four parameter logistic desirability function

Description

Maps a numeric variable to a 0-1 scale with a logistic function.

Usage

d.4pl(x, hill, inflec, des.min = 0, des.max = 1)

Arguments

x

Vector of numeric or integer values.

hill

Hill coefficient. It controls the steepness and direction of the slope. A value greater than zero has a positive slope and a value less than zero has a negative slope. The higher the absolute value, the steeper the slope.

inflec

Inflection point. Is the point on the x-axis where the curvature of the function changes from concave upwards to concave downwards (or vice versa).

des.min, des.max

The lower and upper asymptotes of the function. Defaults to zero and one, respectively.

Details

This function uses a four parameter logistic model to map a numeric variable onto a 0-1 scale. Whether high or low values are deemed desirable can be controlled with the hill parameter; when hill > 0 high values are desirable and when hill < 0 low values are desirable

Note that if the data contain both positive and negative values this function does not provide a monotonic mapping (see example).

Value

Numeric vector of desirability values.

See Also

d.low, d.high

Examples

# High values are desirable
x1 <- seq(80, 120, 0.01)
d1 <- d.4pl(x = x1, hill = 20, inflec = 100)
plot(d1 ~ x1, type="l")

# Low values are desirable (negative slope), with a minimum
# desirability of 0.3
d2 <- d.4pl(x = x1, hill = -30, inflec = 100, des.min=0.3)
plot(d2 ~ x1, type="l", ylim=c(0,1))

# Beware of how the function behaves when the data contain both
# positive and negative values
x2 <- seq(-20, 20, 0.01)
d3 <- d.4pl(x = x2, hill = 20, inflec = 1)
plot(d3 ~ x2, type="l")