Understanding Constraints

     

Constraints can be quite useful if you have tasks that must be associated with a specific date. An example of this might be a meeting, a conference, or some other event. However, if constraints are used incorrectly, they can cause problems with Project's scheduled tasks .

A constraint is a way in which a project manager can control how Project calculates start and finish dates for tasks by associating a specific date with a task, and choosing how the task is related to that date. The constraints you can choose are as follows :

• As Soon As Possible (ASAP)

• As Late As Possible (ALAP)

• Must Start On (MSO)

• Must Finish On (MFO)

• Finish No Earlier Than (FNET)

• Finish No Later Than (FNLT)

• Start No Earlier Than (SNET)

• Start No Later Than (SNLT)

Constraints

Type	Description
ASAP	Flexible constraint that allows Project full control to reschedule the start of the task. This is the default constraint for projects scheduled from the start date.
ALAP	Flexible constraint that allows Project full control to reschedule the finish of the task. This is the default constraint in projects scheduled from the finish date.
MSO	Inflexible constraint that ties the start date of the task to the specified date. Project cannot move the task start earlier or later.
MFO	Inflexible constraint that ties the finish date of the task to the specified date. Project cannot move the task finish earlier or later.
FNET	Constraint that ties the finish of the task to the specified date, but allows the task finish to be moved later in time but NOT earlier.
FNLT	Constraint that ties the finish of the task to the specified date, but allows the task finish to be moved earlier in time but NOT later.
SNET	Constraint that ties the start of the task to the specified date, but allows the task start to be moved later but NOT earlier.
SNLT	Constraint that ties the start of the task to the specified date, but allows the task start to be moved earlier but NOT later.

ASAP and ALAP are flexible constraints, in that they do not restrict how Project schedules the tasks. They are really the absence of a constraint. ASAP is the default constraint for projects that are scheduled from the start date, whereas ALAP is the default for projects scheduled from the finish date.

MSO and MFO are inflexible constraints. They restrict the start or finish of a task to a specific date. These should be used with caution as they limit how Project can reschedule tasks that have predecessor or successor links.

FNET, FNLT, SNET, and SNLT are not completely inflexible, but neither are they completely flexible. They restrict the movement of a task in one direction but not in the other, whereas MFO or MSO constraints restrict in both directions.

An example of how an inflexible constraint (or even one of the semi-flexible constraints) can limit rescheduling can be seen in the "Creating Constraints" section.

Constraints can have unintended consequences regarding how Project schedules your project. The figure shown here shows a common Finish-to-Start link, but it also shows a constraint that changes the way Project is allowed to "flow" the tasks if things change.

The Write Specifications task is linked so that it does not start until Develop Requirements is finished. But it also has a SNET constraint with 5/26 as the Constraint Date. This means that Project will not let this task start before 5/26, but it can start later than 5/26. The outcome of this is that if Develop Requirements finishes one day early, the start date of Write Requirements will not slide back one day.

The key thing to remember is that Project will put a SNET constraint on a task anytime you manually edit the start date of the task (when the project is scheduled from the start date). Although you do have to take manual action to create a MSO or MFO constraint, the creation of SNET or FNET constraints happens anytime you edit the start or finish dates manually.