MSProject Forward/Backward scheduling

[lfl75]

I am getting inconsistent results when I try to use MSProject with both forward scheduling, and I am not sure how MSProject works in this mode to understand how to resolve the issue.

I have a project plan where I am doing normal forward scheduling for may primary tasks. I have other tasks were I need to know when those tasks need to start by to support my primary tasks and am using backward scheduling by creating those tasks as linked tasks that have a constraint type of "Start as late as possible".

Here is an example:

If I have a simple project plan of design, install, test, and deploy, where the install includes installing software and hardware. These tasks are all linked using forward scheduling to tell me when the project will complete. The hardware installation needs to happen at a specific time in the project. In order to do that I have to issue a PO, and receive the equipment to meet the installation date. These are backward scheduled from the install date to see where I would need to issue my PO in order to meet my installation date.

When I set this up I can usually get it to work after much effort and it is often unstable. When it goes unstable the backward scheduled tasks seem to be driving the forward schedule dates. The as "late as possible" constraint is picking a task further down the plan as the date to anchor off of. It seems like the order of linking and assigning constraint type makes a difference. Sometimes temporarily anchoring the plan with a "must start" constraint allows it to work.

Can some one tell me how to achieve my example, or how MSProject treats the "Start as late as possible" constraint in this case.

Thanks

 

[JulieInMaine]

An as late as possible constraint will likely not work for the hardware task. ALAP constrained tasks will align themselves with the latest task in the project. f the hardware installation cannot start based solely on predecessors, you'll need to constrain that task. However, you should be able to link the PO and equipment tasks to hardware with as late as possible constraints.

It's tough to go too much further without more specifics. See the attached file for an sample of my configuration.

 

[lfl75]

Thank you Julie.

In your example I would have a dependency between D3 and T1 that would continue all the way through to DP3. It seems like sometimes it recognizes this dependency and looks at T1 as the latest possible date it could schedule to (based on the forward scheduling through to the end of the project) and sometimes it seems to not recognize it. I was hoping to understand how MSProject looked at it. I have been able to set a fixed constraint type to the install or end of the project to get the backward scheduled portion of the plan to align correctly. The problem with that is that if my plan is adjusted and the install date changes I am locked in.

LFL

 

[JulieInMaine]

Sorry for the delay in replying. I'm afraid I don't quite follow your comment "I would have a dependency between D3 and T1 that would continue all the way through to DP3. Could you possibly post your file? I can't really (possibly) explain what I can't see.

You are correct that constraints need correcting when things change. However, assuming you have only constrained the hardware installation task and driven the rest of the schedule through links, it shouldn't be that awful.

 

[lfl75]

What I meant was that D3 would have been a predecessor to T1, and this would continue all the way through to DP3. This was all forward Scheduling with a constraint of "as soon as possible". The path that was backward scheduled seemed to recognize this path and would work if I had some sort of an anchor along that path, like making DP3 a "must complete on". By anchoring the path the backward schedule portion does not move T1, instead it back schedules from the date established for T1 from the forward schedule. This is how I would want it to work, but it seems to be inconsistent. I am not able to tell what causes it to work or not. I was hoping there were guidelines on how project looks at this.

Thanks

 

[JulieInMaine]

In the example I attached, D3 is an indirect predecessor to T1. I don't think it is necessary to link D3 to T1 but if I do it does not change the schedule - it will merely make T1 dependent upon both task 8 and D3. As task 8 is later D3 is not the driving predecessor.

Again, it would be really helpful for me to help you, if you attached a picture of your file.

What drives task schedules:
Calendars (Project calendar if no resources are assigned, resource calendars if resources are assigned.
Constraints - default is ASAP which is what schedules all tasks to align with the project start date. An ALAP constraint will align itself with the finish date of the project - assuming no successors. If the task has a successor project will "backwards" schedule from the project finish date but schedule the successor to align with the finish date.