convert_age_to_thresh computes the threshold
from a person's age using either the logistic function
or the truncated normal distribution
convert_age_to_thresh(
age,
dist = "logistic",
pop_prev = 0.1,
mid_point = 60,
slope = 1/8,
min_age = 10,
max_age = 90,
lower = stats::qnorm(0.05, lower.tail = F),
upper = Inf
)Arguments
- age
A non-negative number representing the individual's age.
- dist
A string indicating which distribution to use. If dist = "logistic", the logistic function will be used to compute the age of onset. If dist = "normal", the truncated normal distribution will be used instead. Defaults to "logistic".
- pop_prev
Only necessary if dist = "logistic". A positive number representing the overall population prevalence. Must be at most 1. Defaults to 0.1.
- mid_point
Only necessary if dist = "logistic". A positive number representing the mid point logistic function. Defaults to 60.
- slope
Only necessary if dist = "logistic". A number holding the rate of increase. Defaults to 1/8.
- min_age
Only necessary if dist = "normal". A positive number representing the individual's earliest age. Defaults to 10.
- max_age
Only necessary if dist = "normal". A positive number representing the individual's latest age. Must be greater than min_aoo. Defaults to 90.
- lower
Only necessary if dist = "normal". A number representing the lower cutoff point for the truncated normal distribution. Defaults to 1.645 (stats::qnorm(0.05, lower.tail = F)).
- upper
Only necessary if dist = "normal". A number representing the upper cutoff point of the truncated normal distribution. Must be greater or equal to lower. Defaults to Inf.
Value
If age is a positive number and all other necessary arguments are valid,
then convert_age_to_thresh returns a number, which is equal to
the threshold.
Details
Given a person's age, convert_age_to_thresh can be used
to first compute the cumulative incidence rate (cir), which is
then used to compute the threshold using either the
logistic function or the truncated normal distribution.
Under the logistic function, the formula used to compute
the threshold from an individual's age is given by
$$qnorm(pop\_ prev / (1 + exp((mid\_ point - age) * slope)), lower.tail = F)$$,
while it is given by
$$qnorm((1 - (age-min\_ age)/max\_ age) * (pnorm(upper) - pnorm(lower)) + pnorm(lower))$$
under the truncated normal distribution.
