Critical path in MSP

[Dave N]

Hi everyone.

I'm relatively new to using MSP in anger and I have a
major issue with the way it shows the critical path.

If all the tasks on the CP are dependencies of each other
then it's all fine and dandy with nice red tasks to
highlight the CP.

If however you assign no dependencies but have all tasks
assigned to the same resource, then it does not show
these as the CP.

Now my understanding of the definition of the CP is any
tasks that, if delayed, will cause a delay in the
earliest completion time of the project. By definition
if all tasks are assigned to a single resource then a
delay in any task will therefore cause a delay in the
project and so is critical!

Anyone got any thoughts on this as it's quite significant
if I can't easily identify the CP in a large schedule
(currently around 700 lines and growing.)

Dave.

 

[John]

Dave,
Just for reference, Project determines the critical path based on Total
Slack (see Tools/Options/Calculation tab). If your "plan" has no
linkages between tasks, my guess is that the Total Slack values are
quite large and therefore nothing is critical. You have a strange plan
however. If there is only one resource how can none of the tasks be
dependent on one another? Granted the single resource can possibly work
on a couple of tasks simultaneously but for the most part, one task has
to be finished before the next can start. Either that or the Duration
for each task has to equal the Duration of the whole plan and then
everything is critical!

With a single resource, it sounds like you need to do resource leveling.
Basically it takes the priority given to each task and allocates the
resource's time to each task based on the priority. Provided there is a
bar style to show critical tasks, with no task linkages and the whole
project leveled, the last task will be the only critical task because it
is the only one with the default "0" days of Total Slack.

My suggestion, if you expect to see a realistic critical path, link the
tasks in a logical sequence of completion. If you contend that none of
the tasks are dependently related remember this, every task in a valid
project plan MUST have a successor. If nothing else, it must be the end
milestone. Otherwise, why are the tasks being done at all?

Hope this helps.
John

 

[Guest]

John.

The project with a single resource is just an example one
to illustrate my point.

Yes my real project has normal task dependencies and
these are entered as required, but having done all this
there are still cases where what I consider to be the
entire CP is not highlighted.

Further to this if I have to enter non-real dependencies
just to get a CP highlighted then this will firstly take
a lot of time to do in a large project, and secondly be
somewhat going against the reason for using a tool to
help determine when to do certain tasks.

I'll try and give a simple and real example here to show
the point. Say I have 4 tasks, their durations are 3,4,5
& 3 respectively. Tasks 1 & 4 use Resource A and Tasks 2
& 3 use Resource B. Task 2 is a pred. to task 1 and
tasks 1 & 3 are pred. to task 4.

Here all tasks have a successor.

With this MSP shows tasks 3 & 4 as critical but not task
2, however if task 2 slips then the whole project slips.

You'll probably have to enter this in MSP to see it,
shame I can't attach a file here!


Dave.

 

[Jan De Messemaeker]

Hi,

I disagree.
I entered your task ans resources in MSP.
First, without resource leveling, tasks 1, 2 and 4 are critical, not task 3.
When you say task 3 is critical, that means you have performed resource
leveling.

But starting from the assumption that you optimize resoruce usage all the
time, your statement that a delay on task 2 causes a delay on the project is
wrong.

When I introduce (via a constraint) for task 2 a delay lower than its total
slack after leveling (say 1 day) and I re-do Resource Leveling to optimize
my resource usage, Task 3 gets split and the PROJECT END DATE DOES NOT MOVE.
Task 2 is not critical in a resoruce-leveled environment.

Of course when task 2 gets delayed AND resoruce B hasn't started work on
task 3, then your project gets delayed.
But which of the 2 assumptions should Project take? It assumes that you are
actively managing, such that you have your resources work a maximum..

Greetings,

 

[Dave N]

Hi Jan

I see what you are saying and I don't totally disagree,
but you are making a couple of assumptions, firstly you
are assuming that task 3 can be split, but that's not too
important for this.

However more importantly you are assuming that the delay
in task 2 is just a delay in it's starting time. What
will affect the schedule and what makes this task
critical is that it may take longer to execute than
estimated. Tasks 2, 3 & 4 are all critical in that their
total time to complete must not go beyond the estimate.
Task 1 can take longer to complete (upto 2 days longer)
and the schedule is not affected.

Dave.

 

[Steve House]

You are describing what is known as the "Resource Critical Path" which is
distinct from the Critical Path as defined in CPM. The "vanilla" critical
path is defined in terms of the process logic only and that is what MS
Project, and I think all other scheduling software, uses. It ignores
resource dependencies unless you model them through the use of predecessor /
successor linking (IMHO not a good idea). The resource critical path takes
into account both process logic and resource competition among tasks.
Determining the resource critical path and monitoring performance against it
is a manual process and I'm not aware of any scheduling software that has
the capability of calculating it for you. If you think about it, it makes a
certain sense to handle it that way because in your example, MSP has no way
of knowing but what you might have a resource C waiting in the wings whom
you could substitute for B on task 3 if B is still tied up finishing a
delayed finishing task 2 when 3 is due to start, thus preserving 3's
original post-leveling start and consequently the post-leveled critical path
of S->3->4->F.

http://www.allpm.com/article.php?sid=1083

 

[Mark Durrenberger]

What you are looking for is the "Critical chain" a fancy name for the
"resource leveled critical path"

Search the web for "Critical chain" and you'll find more than you wanted to
know...

Mark


--
_________________________________________________________
Mark Durrenberger, PMP
Principal, Oak Associates, Inc, www.oakinc.com
"Advancing the Theory and Practice of Project Management"
________________________________________________________

The nicest thing about NOT planning is that failure
comes as a complete surprise and is not preceded by
a period of worry and depression.

- Sir John Harvey-Jones

 

[Dave N]

Steve.

Thanks for that, I think you confirmed my original
concerns that neither MSP or any other planning apps.
take account of the resources in CPA. They use the
calculation of slack to determine the CP and this is how
you would do it if using pen and paper and AonA or AonN
analysis.

At least now I know for sure!

Cheers.

Dave.

 

[davegb]

If a task can have a 2 day delay in it's start, and therefore, it's
finish, without extending the project end date, it can also have a 2
day extension to it's duration. If it has 2 days or more of Total
Slack, neither condition effects the critical path.
There are actually 3 definitions of CP, each superceding the previous,
when done in the logical order of a CP analysis. The first is the
longest path, the second is the path along which Early Start = Late
Start, and the 3rd is the path with 0 Total Slack. Since Total Slack =
Late Start - Early Start, these are the same paths. Since the along
the longest path, Late Start will always equal Early Start, the three
are the same.
The second biggest problem people have in using Project is not
understanding these relationships and the fact that they need Schedule
Continuity to make it all work.
Hope this helps.

David G. Bellamy
Bellamy Consulting

 

[Dave N]

Mark.

Yep, looks like you're right, so looks like it's off to
Amazon to buy a book on critical chain!

Thanks.

Dave.

 

[Steve House]

Yep - MSP is based on classical CPM with Action-on-Node.
--
Steve House [MVP]
MS Project Trainer/Consultant
Visit http://www.mvps.org/project/faqs.htm for the FAQs

 

[Mark Durrenberger]

Do not buy "Critical Chain" by Goldratt - it is a thinly veiled attempt to
sell consulting services - gives you just enough info to want to call him
and ask more questions...

I wish I could remember the author to recommend... Sorry.
Mark

ps before buying any books - see what is free on the web

fwiw - a lot of Critical chain PM is general good PM practices - many of the
CC people will claim that no one ever thought of it before but their
memories are either short or selective for marketing purposes...



--
_________________________________________________________
Mark Durrenberger, PMP
Principal, Oak Associates, Inc, www.oakinc.com
"Advancing the Theory and Practice of Project Management"
________________________________________________________

The nicest thing about NOT planning is that failure
comes as a complete surprise and is not preceded by
a period of worry and depression.

- Sir John Harvey-Jones

 

[JackD]

Do not buy "Critical Chain" by Goldratt - it is a thinly veiled attempt to
sell consulting services - gives you just enough info to want to call him
and ask more questions...

I wish I could remember the author to recommend... Sorry.
Mark

ps before buying any books - see what is free on the web

fwiw - a lot of Critical chain PM is general good PM practices - many of the
CC people will claim that no one ever thought of it before but their
memories are either short or selective for marketing purposes...

How are you going to sell the same old stuff unless you give it a new name?

-Jack "New and Improved"

 

[Scott Button]

I recommend the book "Critical Chain Project Management" by Lawrence P.
Leach. Also, a couple of good websites:
http://www.pdinstitute.com/

http://www.focusedperformance.com/