Confused about total slack

[fretnoise]

I can't figure out how MS Project calculates Total Slack. For most
cases, it's
LS - ES. However, that doesn't always hold. Have googled on this for
hours but can't
find an asnwer.

Here's a simple example:

- add two tasks to a new project, "a", "b"
- make "b" a predecessor of "a"
- add constraint MSO 6/1/06 on "b"
- schedule forward with start date 6/12/06

MS Project 2003 calulcates the following (TS=total slack, FS=free
slack, SS=start slack)

(1) task a: ES=6/12 EF=6/12 LS=5/31 LF=5/31 TS=-8 FS=0 SS=-8

(2) task b: ES=6/1 EF=6/1 LS=6/1 LF=6/1 TS=-8 FS=0 SS=-8

Both tasks have the same slack, even though they have different values
for
LS-ES or LF-EF. Task b has the same dates for ES/EF/LS/LF yet the TS is
not 0.
This shows that TS cannot be computed from just ES/EF/LS/LF.

How does MS Project exactly calculate Total Slack? Anybody have a
formula (the
english explanation in Microsoft Project Help isn't very complete)

Thank's very much!

 

[Jan De Messemaeker]

Hi,

Did exactly what you did and both tasks have TS of 0.

 

[davegb]

Interesting, how we can get 3 different results from such a simple
calculation!

Assuming the values you posted are the ones Project yields, for task a,
I get 5/31-6/12 = -8d TS. For b, 6/1-6/1 = 0d TS. I get the same result
when I use LF and EF for both.

Hi,

Did exactly what you did and both tasks have TS of 0.

Task b does not have the same values for ES. a's ES is 6/12. b's ES is
6/1, if you've quoted them correctly just above. (I don't have Project
here to check what you've said it shows for these values.)

I wouldn't count for a second the Project has it right! Calculation of
these dates, and therefor TS, gets complicated when you account for SS,
FF and SF relationships and lead and lag times. I would have no problem
believing that Project may, in some cases, have it wrong. Look how long
it's taken MS to get EV correct, at least up to a point. By definition,
TS = LS - ES = LF - EF. If Project shows it otherwise in any situation,
there's an error in Project.

As for Start Slack, never heard of it before. How is it calculated and
what does it tell you?

 

[fretnoise]

Did exactly what you did and both tasks have TS of 0.

Thank's Jan, that's odd.

I've just tried it again, and it gives me -8. I'm running version
11.0.2003.0816.15. Maybe you're running a different version?

 

[fretnoise]

Hi Dave, thank's for your reply.

Task b does not have the same values for ES. a's ES is 6/12. b's ES is
6/1, if you've quoted them correctly just above. (I don't have Project
here to check what you've said it shows for these values.)

I did not mean that b's ES is the same as a's ES. I meant that
b's values for ES, EF, LS, LF are all 6/1, yet its reported TS != 0.

I wouldn't count for a second the Project has it right! ...
As for Start Slack, never heard of it before. How is it calculated and
what does it tell you?

According to MS Project 2003 help, the following should hold:
o Start Slack = duration between ES and LS
o Finish Slack = duration between EF and LF
o Free Slack = Min(Start Slack, Finish Slack)

But those formulas don't hold for my scenario.Either MSP calculates it
wrong, or the help documentation is wrong, or both.

What version of MSP are you running? Hopefully we all run different
builds, so
that would explain different results.

 

[davegb]

Hi Dave, thank's for your reply.


I did not mean that b's ES is the same as a's ES. I meant that
b's values for ES, EF, LS, LF are all 6/1, yet its reported TS != 0.


According to MS Project 2003 help, the following should hold:
o Start Slack = duration between ES and LS
o Finish Slack = duration between EF and LF
o Free Slack = Min(Start Slack, Finish Slack)

The difference between ES and EF and the difference between LS and LF
should always be the same since EF = ES + Dur. and LS = LF - Dur. That
number is Total Slack.

Free Slack is the difference between the earliest ES of any successor
task and the EF of the task you're calculating the FS of. I think
someone at MS needs to go back to school.