PERT analysis on Work

[Andrew K]

Is there a way to do a 3-point PERT analysis on Work estimates?

 

[Jack Dahlgren]

Not that I am aware of.
PERT has been discredited as being statistically valid so I wouldn't expect
that they will update this feature in the future.
You could conceivably do a monte carlo simulation of work. The results would
be somewhat interesting...
For some code to start with you can look at my free monte carlo simulation
macro here:

http://masamiki.com/project/blackjack.htm

-Jack Dahlgren

 

[Jim Aksel]

Jack - do you have a reference on this discredit?
I know the 1-4-1 issue is sometimes challenged, but Project allows you to
adjust the weighting. If you think about it, the requirement that it total 6
is not restrictive, you can produce any three values you want and normalize
it to 6.

Monte Carlo seems to work better than 1-4-1 weighting, although it still
requires a triple point estimate and assumptions on probability
distributions which are also open to challenge. When using Monte Carlo I
give myself the widest range reasonable possible with the Triangular
distribution becuase of its larger standard deviation.
--
If this post was helpful, please consider rating it.

Jim

Visit http://project.mvps.org/ for FAQs and more information
about Microsoft Project

 

[Andrew K]

There does seem to be a workaround: Do the PERT exercise on duration and
then copy the results from the Duration field to the Work field. (copy-paste
results in an error, so I type). Is there a hidden gotcha here?

 

[davegb]

Jack - do you have a reference on this discredit?
I know the 1-4-1 issue is sometimes challenged, but Project allows you to
adjust the weighting. If you think about it, the requirement that it total 6
is not restrictive, you can produce any three values you want and normalize
it to 6.

Monte Carlo seems to work better than 1-4-1 weighting, although it still
requires a triple point estimate and assumptions on probability
distributions which are also open to challenge. When using Monte Carlo I
give myself the widest range reasonable possible with the Triangular
distribution becuase of its larger standard deviation.
--
If this post was helpful, please consider rating it.

Jim

Visithttp://project.mvps.org/for FAQs and more information
about Microsoft Project






- Show quoted text -

I have looked for the reference I saw years ago showing that PERT is
statistically unsound. I can't find it. But the reasoning was that it
only accounts for the different probable durations of each individual
task, and ignored the cumulative risk along each path through the
project. The cumulative risk being far more significant than the
individual risk. So what he was saying is that PERT accounts for 10%
of the probable deviation and ignores 90% (percentages are made up,
don't know what they really might be, but you get the idea).

For example, one path through the project has 5 tasks. 4 have little
schedule risk, and when you do a PERT analysis, you find you're adding
a day or so the the 20 day duration of that path. Another path may
have 5 tasks, but all 5 have very high schedule risk, and you might,
by PERT analysis, add 5 days to the 20 day total duration. But since
all 5 tasks on this path have high SDs, the real risk of schedule
overrun on this path is much greater than on the other path, because
one task slipping early in this path can cause all the others to go
way over Expected duration, say double the 20 days. So, much more than
5 days should be added to it's probable duration to account for this.
PERT simply doesn't do this. Monte Carlo does.

The article I read was by a statistician who had studied PERT, had
very credible credentials, and made a lot of sense. I have steered my
clients/students away from it ever since. It's better than nothing,
but not much!

Hope this helps in your world.

 

[Jack Dahlgren]

Sure,
What do the results "Mean"?
You can do all sorts of math, but is the method statistically valid?

-Jack

 

[Steve House]

The gotcha is that while Work and Duration are related to each other, it's
only under specific circumstances that they are equal to each other. Your
idea would work when there's one resource assigned 100% but for multiple
resources or assignments at other than 100% it breaks down. (Just talking
the mechanics of "working the software" here - whether the results of such
an approach would actually mean anything is another matter, as others have
discussed.)

 

[Andrew K]

Yes, I know. I realize I've figured out how to use Project as a calculator

All told, assuming PERT has validity, I may as well do the calculations in
Excel and Copy-Paste the values into the Work field.

Many thanks, as always.

The gotcha is that while Work and Duration are related to each other, it's
only under specific circumstances that they are equal to each other. Your
idea would work when there's one resource assigned 100% but for multiple
resources or assignments at other than 100% it breaks down. (Just talking
the mechanics of "working the software" here - whether the results of such
an approach would actually mean anything is another matter, as others have
discussed.)
--
Steve House [Project MVP]
MS Project Trainer & Consultant
Visit http://www.mvps.org/project/faqs.htm for the FAQs

There does seem to be a workaround: Do the PERT exercise on duration and
then copy the results from the Duration field to the Work field.
(copy-paste
results in an error, so I type). Is there a hidden gotcha here?

 

[davegb]

Yes, I know. I realize I've figured out how to use Project as a calculator

All told, assuming PERT has validity, I may as well do the calculations in
Excel and Copy-Paste the values into the Work field.

And you would assume that because?

 

[Robert]

Yes.
See Brian Kennemer's article in the MPA hardcopy newsletter about a year
ago. If you can't find it , let me know, I have it somewhere