Missing critical path

[Henrik Jensen]

The case
Task A has a finish-to-start relationship to Task B
Task A starts on january 1., duration 2 days, assignment = Resource 1
Task B, duration 5 days, assignment = Resource 2


The problem
I can't figure out why the critical path dissapears when a resource,
assigned to Task B, have a nonworking day on january 3.
If the resource have a nonworking day on january 4. the critical path exists.

Please help med understand!

 

[John]

The case
Task A has a finish-to-start relationship to Task B
Task A starts on january 1., duration 2 days, assignment = Resource 1
Task B, duration 5 days, assignment = Resource 2


The problem
I can't figure out why the critical path dissapears when a resource,
assigned to Task B, have a nonworking day on january 3.
If the resource have a nonworking day on january 4. the critical path exists.

Please help med understand!

Henrik,
That's because a task is on the critical path if it has zero total slack
(assuming the default setting). If resource 2 is off on 3 January, then
task A actually has one day of total slack so it is not critical. Think
of it this way, since resource 2 is off on 3 January, that effectively
gives resource 1 one extra day to finish task A before resource 2 will
be available to work on task B.

Hope this helps.
John
Project MVP

 

[Henrik Jensen]

Hi John,

Thanks for your answer!
I fully understand your answer, but i find it strange because if you look at
my example, the finish date will be the same anyway.

I find it confusing that if you have a project with 20000 tasks, and the
above example appears between the last two tasks, only one task will be shown
as critical.

As a project manager it is important to focus on the critical path, and it
will not be easy if you only are shown one critical path. In fact this is
what many project managers tell me.

Is there anything i can do about it?

//Henrik

 

[Jack Dahlgren]

You can set the definition of "Critical" to something other than 0 days
slack.
Go to 'Tools" menu, "Options", "Calculation" tab and set "Tasks are critical
if slack is less than or equal to " to an appropriate value. I'd say a few
days would probably be OK. Also on the same tab, check the "Calculate
Multiple Paths" option.

Personally, I'd just display the slack column and work from it directly to
get an idea , or modify the barstyles so that they show slack (set the
barstyle to draw a bar from finish to late finish)

-Jack Dahlgren

 

[Henrik Jensen]

Hi Jack,

I know about the "Tasks are critical if slack is less than or equal to"
setting, but what is the appropriate value?

You could also insert a milestone as the last task and set its start or
finish date equal to the project finish date minus an appropriate number of
days, it just look funny on the gantt chart, but it works the same way.

I have been teaching Microsoft Project for almost 20 years, and done project
consulting for the same amount of years. My recommendation to Microsoft is to
implement a check box saying "Display critical path ignoring nonworking time
on resources"...or something like that. Do you agree or don't you find this a
problem?

Henrik Jensen
MCP, MCP+SB, MCAD, MCSD, MCTS, MCPD, MCITP, MBSS, MBSP, MCT

 

[Jack Dahlgren]

Henrik,

The appropriate value for the total slack question is wide open to
judgement. How long is the project? How often do you check the schedule?
What is your confidence in the precision of the estimates? Are you just
looking at the critical path and ignoring risk? Are you doing any
probabilistic scheduling (monte carlo simulations perhaps)? Are you
buffering the schedule in one way or another?

That is why I'd ignore it and just look at how total slack plays out across
the schedule. The whole idea of a deterministic critical path in the face of
uncertain task durations doesn't seem to hold up to much scrutiny. The
schedule is just a model. All models are wrong. Once you accept that, you
can get to work putting them to use.

-Jack Dahlgren

 

[Henrik Jensen]

Hi Jack,

I totally agree with you.

The reason i bring this "problem" up is because many people find it annoying
that they cannot see the critical path, and they stop registering nonworking
days on resources because of that.

If you set the value of "Tasks are critical if slack is less than or equal
to", and you leave the project to another project manager, he might not find
out (at first) that you have set a value to 10 days.

As you wrote earlier it might be best to show the total slack column, or
indicate slack on tasks in the Gantt Chart.

Anyway thanks Jack ;-)

//Henrik