This is the distribution function of the sigmaami transformation.
A size-n sample of blood pressure average errors (average of
the difference between three device measurements
and the three corresponding reference readings)
is drawn from a normal distribution
with mean mu and standard deviation std.dev.
The r.v. sigmaami results from transforming \(xbar\), the
sample mean, assuming the proportion of drawn errors lie in
(-delta, delta)
with probability ptolerror, that is
sigmaami\(=sigmaami(xbar)\).