How do resources/fixed work affect critical path?

[Kevin]

I've been playing around with my project file, and I
realized now why my critical path does not show the way I
would expect. All of my tasks are setup as fixed work
tasks. I then assigned a single resource to each of them
at 60% time (this is basically how our projects work).

After making these resource assignments, most of the tasks
I would expect to show in the critical path are no longer
there. Can someone please explain how the critical path
is calculated when it comes to resources and fixed-work
tasks?

Thanks for any help- I'm not finding much rhyme or reason
to this.

 

[Mike Glen]

Hi Kevin,

Welcome to this Microsoft Project newsgroup

Could you see the critical path before you assigned any resources? Have you
levelled the resources? A critical task, by definition has zero slack (have
a look at the Slack column) and the critical path is the shortest path
through the network joining the critical tasks ( you can make it red via the
Gantt Chart Wizard).

FAQs, companion products and other useful Project information can be seen at
this web address: http://www.mvps.org/project/

Hope this helps - please let us know how you get on

Mike Glen
Project MVP

 

[Cristina Pupper]

Hi-

By definition, Critical Path analysis does not take into consideration
resource availability & leveling, a huge limitation. You would have to look
at Critical Chain methodology if you wanted to take resource availability
into consideration when finding the longest 'path' through your project.
Critical Chain is available through add-ins to MS Project. ProChain is the
only one I am familiar with.


Hope this helps!

 

[Kevin]

Thanks for the welcome Mike

I could see the critical path before I added resources. I
added the slack column as you suggested, and the result
was interesting. I noticed tons of slack in many of those
columns, but I'm not quite sure why.

When I created my tasks, I set them up as Fixed Work, and
entered the values into the Work column (so that the
durations would self-calculate). I then added a single
resource to the tasks with the units column at 60%.
Therefore, the duration is longer than the work. Could
this be what's creating the slack?

 

[Steve House]

The task type (fixed work, etc) and resource assignment levels don't
directly affect the slack time calculation, thus critical path, at all
except as they may impact task duration. Slack is defined as the amount of
time a task could be delayed without a: delaying a successor task (free
slack), b: causing a downline successor task to complete after a deadline
(MSP 2000 or later), or c: delaying the project completion (total slack).
(Project 2000 and above considers total slack to be defined by either
condition b or c, whichever is less.) A critical task is one whose total
slack is zero. Task durations, constraints, deadlines, and the web of
dependency links are the driving force. Project completion is not the
finish of the last task in the list nor of a finish milestone unless all
tasks are somehow linked to be a predecessor to it, rather it's the finish
date of the latest finishing task. So from a practical standpoint you could
say a task is critical if a delay in its completion somehow, directly or
indirectly, will cause the last-to-finish task in the project to finish
later than it's currently scheduled.

To see how it works in MSP, create a project file and enter tasks
A,B,C,D,E,F,G,H,I,J,K,L,M,N,O,P. Make their durations
1,2,3,4,5,6,7,8,9,8,7,6,5,4,3,2 days respectivly. Do not link any tasks.
Right click the Gantt chart and format it as the critical Gantt using the
wizard. You'll find only the 9 day task is shown in red, critical. Why?
It's finish is the project finish and any delay to it delays the project
completion. Link the 1st four tasks. They'll be red and the 9 day task
changes back to blue. Why? Because now the chain ABCD is longer than 9
days, making task D is the last to finish and the first three are in the
chain leading to it, so if something happens to delay on of them, D will
finish even later. Task I is no longer the latest finishing task nor in the
chain leading to the latest one so it's no longer critical.

 

[Mike Glen]

Hi Kevin,

I think it's more a product of the linking of tasks. As I don't know your
skill level: would you confirm that you've scheduled your project in line
with the guidance given in FAQ Item: 42. Guide to Network Analysis. FAQs,
companion products and other useful Project information can be seen at this
web address: http://www.mvps.org/project/

Mike Glen
Project MVP

 

[Kevin]

The problem did turn out to be in my linking. I had one
task that had 3 predecessors, one of which was a SS link.
Once I deleted the predecessors from that task, my
critical path "showed up" the way I would have expected it
to. It just so happened that I also added my resources at
the same time as that one faulty link, so that's why I
made that association. It's kind of scary how easy it is
to screw up a project with a bad link- and it certainly
doesn't help that Project doesn't have a decent Undo
system!

Thanks for all your help. It steered me in the right
direction.

 

[Mike Glen]

You're welcome, Kevin

Mike Glen
MS Project MVP