Hello everyone,
I am trying to define a PERT distribution "by hand" by transforming a classic
beta distribution - as I would like to understand where the shape parameter
(default value of 4) comes from in the mc2d package.
The help page says
"The PERT distribution is a beta distribution extended to the domain [min, max]
with mean mu = (min + shape * mode + max)/(shape + 2)"
At the same time, we know that a 4 parameter beta distribution's mean can be
expressed as
mu = (alpha * max + beta * min)/(alpha + beta)
So using the equality between the 2 expressions of mu I can find the shape
parameter as a function of alpha and beta. But if I want to set the shape
parameter to 4, how do I define alpha and beta? All I know is that my
distribution is unimodal and positively skewed.
In other words, is there a way to find the values of the 2 shape parameters of
the "underlying beta distribution" by setting the min, max, mode, maybe some
constraints on alpha and beta (alpha < beta; >1) and R-defined unique shape
parameter?
I think maybe I am missing something here.
Many thanks
________________________________
This message and any attachments contain information that may be RMS Inc.
confidential and/or privileged. If you are not the intended recipient (or
authorized to receive for the intended recipient), and have received this
message in error, any use, disclosure or distribution is strictly prohibited.
If you have received this message in error, please notify the sender
immediately by replying to the e-mail and permanently deleting the message from
your computer and/or storage system.
[[alternative HTML version deleted]]
______________________________________________
R-help@r-project.org mailing list
https://stat.ethz.ch/mailman/listinfo/r-help
PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.
