Microsoft Project 2010 : Fine-Tuning Task Details (part 2) - Setting Task Constraints

2/10/2014 8:52:07 PM

2. Setting Task Constraints

Every task that you enter into Project has some type of constraint applied to it. A constraint controls the start or finish date of a task and the degree to which that task can be rescheduled. There are three categories of constraints:

Flexible constraints. Project can change the start and finish dates of a task. The default constraint type in Project is that tasks start as soon as possible. This type of flexible constraint is called As Soon As Possible, or ASAP for short. No constraint date is associated with flexible constraints. Project does not display any special indicator in the Indicators column for flexible constraints.
Inflexible constraints. A task must begin or end on a certain date. For example, you can specify that a task must end on November 9, 2012. Inflexible constraints are sometimes called hard constraints. When an inflexible constraint has been applied to a task, Project displays a special indicator in the Indicators column. You can point to a constraint indicator, and the constraint details will appear in a ScreenTip.
Semi-flexible constraints. A task has a start or finish date boundary. However, within that boundary, Project has the scheduling flexibility to change the start and finish dates of a task. For example, let’s say a task must finish no later than June 15, 2012. However, the task could finish before this date. Semi-flexible constraints are sometimes called soft or moderate constraints. When a semi-flexible constraint has been applied to a task, Project displays a special indicator in the Indicators column.

In total, there are eight types of task constraints.

This constraint category	Includes these constraint types	And means
Flexible	As Soon As Possible (ASAP)	Project will schedule a task to occur as soon as it can occur. This is the default constraint type applied to all new tasks when scheduling from the project start date. There is no constraint date for an ASAP constraint.
	As Late As Possible (ALAP)	Project will schedule a task to occur as late as it can occur. This is the default constraint type applied to all new tasks when scheduling from the project finish date. There is no constraint date for an ALAP constraint.
Semi-flexible	Start No Earlier Than (SNET)	Project will schedule a task to start on or after the constraint date that you specify. Use this constraint type to ensure that a task will not start before a specific date.
	Start No Later Than (SNLT)	Project will schedule a task to start on or before the constraint date that you specify. Use this constraint type to ensure that a task will not start after a specific date.
	Finish No Earlier Than (FNET)	Project will schedule a task to finish on or after the constraint date that you specify. Use this constraint type to ensure that a task will not finish before a specific date.
	Finish No Later Than (FNLT)	Project will schedule a task to finish on or before the constraint date that you specify. Use this constraint type to ensure that a task will not finish after a specific date.
Inflexible	Must Start On (MSO)	Project will schedule a task to start on the constraint date that you specify. Use this constraint type to ensure that a task will start on an exact date.
	Must Finish On (MFO)	Project will schedule a task to finish on the constraint date that you specify. Use this constraint type to ensure that a task will finish on an exact date.

These three constraint categories have very different effects on the scheduling of tasks:

Flexible constraints, such as ASAP, allow tasks to be scheduled without any limitations other than their predecessor and successor relationships, and the project’s start date (for ASAP task constraints) or finish date (for ALAP task constraints). No fixed start or end dates are imposed by these constraint types. Use these constraint types whenever possible.
Semi-flexible constraints, such as Start No Earlier Than or Start No Later Than, limit the rescheduling of a task within the date boundary that you specify.
Inflexible constraints, such as Must Start On, prevent the rescheduling of a task. Use these constraint types only when absolutely necessary.

Note

The type of constraint that you apply to the tasks in your projects depends on what you need from Project. You should use inflexible constraints only if the start or finish date of a task is fixed by factors beyond the control of the project team. Examples of such tasks include handoffs to clients and the end of a funding period. For tasks without such limitations, you should use flexible constraints. Flexible constraints provide the most discretion in adjusting start and finish dates, and they allow Project to adjust dates if your project plan changes. For example, if you have used ASAP constraints and the duration of a predecessor task changes from four days to two days, Project adjusts, or “pulls in,” the start and finish dates of all successor tasks. However, if a successor task had an inflexible constraint applied, Project could not adjust its start or finish dates.

In this exercise, you apply a constraint to a task.

Select the name of task 16, Proofread and index.
On the Task tab, in the Editing group, click Scroll to Task.

Tip

To select a task quickly, even a task you can’t see in the current view, press Ctrl+G, and in the ID field of the Go To dialog box, enter a task number, and then click OK.

This task requires the children’s book author, Tad Orman, to proofread his book at a certain stage in the design process. However, Tad has informed you that due to his travel schedule, he will be unable to start his review before July 13—later than currently scheduled.
On the Task tab, in the Properties group, click Information.
In the Task Information dialog box, click the Advanced tab.
In the Constraint Type box, select Start No Earlier Than.
In the Constraint Date box, type or select 7/13/12, and then click OK.

Project applies a Start No Earlier Than constraint to the task, and a constraint icon appears in the Indicators column. You can point to the icon to see the constraint details in a ScreenTip.

Because this constraint affects the scheduling of the task, the Task Inspector pane now includes the constraint details.

Task 16 is rescheduled to start on July 13 instead of July 11. All tasks that depend on task 16 are also rescheduled. One way to view this rescheduling is by the light blue change highlighting that Project applies to the Start and Finish dates of the successor tasks of task 16. Because the durations of the 1st Pages review and Design and Production summary tasks were also changed by applying the constraint to task 16, the Duration and Finish fields for the those summary tasks are also highlighted. Change highlighting remains visible until you perform another editing action or save the file, and it is an effective visual way to see the broader effects of your specific actions in your schedule.
Click the Close button (the “X” button in the upper-right corner) on the Task Inspector pane.

Here are a few other things to keep in mind when applying constraints to tasks:

Note

You cannot set constraints on manually scheduled tasks. The following issues apply only to automatically scheduled tasks.

Entering a Finish date for a task (for example, in the Finish column) applies a Finish No Earlier Than constraint to the task.
Entering a Start date for a task (for example, in the Start column) or dragging a Gantt bar directly on the Gantt chart applies a Start No Earlier Than constraint to the task.
In many cases, entering a deadline date is a preferable alternative to entering a semi-flexible or inflexible constraint.
Unless you specify a time, Project schedules a constraint date’s start or finish time using the Default Start Time or Default End Time values on the Schedule tab of the Project Options dialog box (to open this dialog box, on the File tab, click Options). In this project, the default start time is 8 A.M. If you want a constrained task to be scheduled to start at a different time, enter that time along with the start date. For example, if you want to schedule a task to start at 10 A.M. on July 12, enter 7/12/12 10AM in the Start field.
To remove a constraint, first select the task or tasks and, on the Task tab, in the Properties group, click Information. In the Task Information dialog box, click the Advanced tab. In the Constraint Type box, select As Soon As Possible or (if scheduling from the project finish date) As Late As Possible.
If you must apply semi-flexible or inflexible constraints to tasks in addition to task relationships, you might create what is called negative slack. For example, assume that you have a successor task that has a finish-to-start relationship with its predecessor task. If you entered a Must Start On constraint on the successor task earlier than the finish date of the predecessor task, this would result in negative slack and a scheduling conflict. By default, the constraint date applied to the successor task will override the relationship. However, if you prefer, you can set Project to honor relationships over constraints. On the File tab, click Options, and in the Project Options dialog box, click the Schedule tab. Clear the Tasks Will Always Honor Their Constraint Dates check box.
If you must schedule a project from a finish date rather than a start date, some constraint behaviors change. For example, the As Late As Possible constraint type, rather than As Soon As Possible, becomes the default for new tasks. You should pay close attention to constraints when scheduling from a finish date to make sure that they create the effect that you intend.