July 2005

By Gus Cicala, Project Product Advisory Council

Applies to:
Microsoft Office Project 2003

In this article

What should you look for if a date does not seem right?

How can constraints restrict my ability to link tasks?

How do task links override a date that I enter?

How do linked summary tasks affect my ability to set a task date?

How does an Actual Start date affect a task that is linked?

When Calculate is set to Manual, how are task dates affected?

One of the strengths of automated project management tools is their ability to recalculate and adjust dates for the project manager. If a predecessor (predecessor: A task that must start or finish before another task can start or finish.) task finishes late, Project moves the successor (successor: A task that cannot start or finish until another task starts or finishes.) (linked) task to a later date for you. While these automatic scheduling features are usually considered helpful, mastering these features requires a complete understanding of how they are designed to work. Project is not misbehaving, you just need to understand what Project assumes when it schedules tasks.

In this article, we will explore how the scheduling algorithms in Project use links, constraints (constraint: A restriction set on the start or finish date of a task. You can specify that a task must start on or finish no later than a particular date. Constraints can be flexible [not tied to a specific date] or inflexible [tied to a specific date].), preset options, and dates to automatically set your task start and finish dates.

What should you look for if a date does not seem right?

If you try to link a task or change the date of a task and Project does not seem to be cooperating, chances are that Project is doing something that makes sense. The trick is to understand why Project is doing what it is doing. There are five situations to look for if a recalculated task date does not seem to be correct:

• The task has a hard constraint, such as Start No Earlier Than (SNET) or Must Start On (MSO). A hard constraint will sometimes override the link between the tasks.
• The task is linked to another task.
• The task is indented below a summary task (summary task: A task that is made up of subtasks and summarizes those subtasks. Use outlining to create summary tasks. Project automatically determines summary task information [such as duration and cost] by using information from the subtasks.) that is linked.
• The task has an Actual Start date.
• The Calculate option is set to Manual.

To spot potential problems when you make a scheduling change, make sure that you always receive Project messages and advice:

1. On the Tools menu, click Options.
2. On the Schedule tab, make sure that the Show scheduling messages check box is selected.
3. On the General tab, make sure that the Advice from Planning Wizard check box and the three check boxes just under it are all selected.

With these options set, Project will present scheduling and Planning Wizard messages as needed to warn you about changes you make that either cannot be accommodated or may have an adverse affect on your schedule.

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How can constraints restrict my ability to link tasks?

Now we will look at how task constraints can make linking tasks and setting successor start dates a little confusing. If you enter a start date for a task, Project will change the default constraint from As Soon As Possible (ASAP) to Start No Earlier Than (SNET) for the date you entered.

How will this constraint affect your task links and successor start dates? Suppose you enter 2/9/06 for Task 17, and then link Task 16 (the predecessor) to Task 17 (the successor) with a finish-to-start (FS) relationship. If Task 16 ends on 2/6/06, you might expect Task 17 to start on 2/7/06. What you might have expected does not happen because Project honors the Start No Earlier Than (SNET) constraint and keeps Task 17 scheduled for 2/9/06. You now have what appears to be a 2-day lag between Task 16 and Task 17.

To see what is really causing the delay between the two tasks, double-click Task 17, and then click the Advanced tab in the Task Information dialog box. You will see that the task has a Start No Earlier Than (SNET) constraint which is overriding the link between the two tasks. In this case, the link does not affect how Task 17 is scheduled.

However, if you look at the Lag field for these linked tasks, it is set to zero days (0d).

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How do task links override a date that I enter?

The opposite effect can sometimes happen to linked tasks. For example, you enter a start date for a task and Project changes the date to what it was before you entered it. In the following figure, Task 2 and Task 3 are linked. Notice that Task 3 is scheduled to begin As Soon As Possible (ASAP).

If you enter a start date of 1/31/06 for Task 3, a Planning Wizard notifies you that if you really want to change the start date for Task 3 to 1/31/06, Project will need to remove the link between the two tasks.

If you decide to proceed and remove the link, the link between Task 2 and Task 3 will disappear, and Task 3 will show a start date of 1/31/06 with a Start No Earlier Than (SNET) constraint.

If, however, you have selected Don't tell me about this again in the Planning Wizard, Project will honor the link between the two tasks, and the start date for Task 2 will remain set to 2/2/06. Note that entering a date for Task 3 changes the default constraint from As Soon As Possible (ASAP) to Start No Earlier Than (SNET) on 1/31/06. In this case, the task link appears to have won the link-versus-constraint battle. In fact, the Start No Earlier Than (SNET) constraint of 1/31/06 is still set, but the link causes Task 3 to start later, which makes sense for this Start No Earlier Than (SNET) constraint.

The easiest way to understand this example is to remember a simple rule. If a task has a Start No Earlier Than (SNET) constraint of a specific date, it cannot start earlier than that date, but it can start later.

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How do linked summary tasks affect my ability to set a task date?

One of the more difficult restrictions to understand is how linked summary tasks (summary task: A task that is made up of subtasks and summarizes those subtasks. Use outlining to create summary tasks. Project automatically determines summary task information [such as duration and cost] by using information from the subtasks.) affect your ability to set a start date for a subordinate task that is related to that linked summary task. If you link the Phase 1 and Phase 2 summary tasks, none of the tasks in Phase 2 can start before all of the tasks in Phase 1 are complete.

Suppose Phase 1 ends on 2/2/06 and Phase 2 is a successor to Phase 1. If you enter a start date earlier than 2/2/06 for any task in Phase 2, Project changes the date back to what it was. This situation can be hard to spot, because you may not realize that the tasks in Phase 2 are part of a linked summary task (Phase 2), unless you specifically look for the link between Phase 1 and Phase 2. Notice in the following figure that Task 7 has a Start No Earlier Than (SNET) constraint on 1/26/06, but Project keeps the start date of 2/7/06. This occurs because Phase 1 does not end until 2/2/06 and it has a finish-to-start (FS) task dependency on Phase 2.

This situation may be even more difficult to see when the summary task does not appear on the same screen as the detail task that you are trying to update. This may occur when a task is toward the end of a large project phase, so the related summary task may appear many tasks higher in the work breakdown structure (WBS) (WBS: A hierarchical structure that is used to organize tasks for reporting schedules and tracking costs. With Project, you can represent the work breakdown structure by using task IDs or by assigning your own WBS code to each task.).

It is also difficult to see this situation when a task is more than one outline level (outline level: The number of levels that a task is indented from the top level of the outline. You can indent tasks up to 65,000 levels in Project.) lower than the related summary task. For this reason, you may have to view every summary task in the WBS to find the linked summary task that is restricting you from modifying a task's start date.

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How does an Actual Start date affect a task that is linked?

Sometimes a task link and the associated start date for a task may appear to misbehave because of a task which contains an Actual Start date. The following figure shows that Task 8 appears to have a negative lag which causes it to start before the start of Task 7. Further investigation shows that Task 8 has a constraint of As Soon As Possible (ASAP) and the lag between Task 7 and Task 8 is zero days (0d). So why does Task 8 begin before Task 7?

If you look at the Task Details Form in the lower pane, you see that Task 8 has an Actual Start date of 11/28/05, which overrides the predecessor link to Task 7. This may occur because you entered an Actual Start date (which means the task has actually begun), or because the task's % Complete field is greater than 0 (which also means the task has actually begun).

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When Calculate is set to Manual, how are task dates affected?

By default, Project performs calculations automatically. However, if you have a very large project, you may want to specify that Project perform most calculations only when you click Calculate Now, as automatic calculations can take a long time. Or you may want to perform calculations manually (though this is not recommended). To change your calculation options, on the Tools menu, click Options, and then click the Calculation tab.

Click Manual for the Calculation mode, and then click the Calculate Now button.

Several things stop happening when the calculation mode is set to Manual. Suddenly, it is not safe to assume that links between tasks will be presented correctly on the Gantt Chart. In the following figure, Tasks 17 through 19 are all linked, but the start date for each of these tasks is still 3/1/06, and the links all appear to have negative lags. If you look closely at the status bar at the bottom of the screen, you will see the word Calculate.

Project is warning you that there is data on your screen that may not be presented correctly. To fix this problem, either press F9 when Calculate appears at the bottom of the screen, or change the Calculation mode option to Automatic.

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Conclusion

If you think a date or a link is acting strangely, do not give up! Project is doing what it is designed to do and what it believes you want it to do. Once you understand how Project sets dates, you will agree with it.

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About the author

Gus Cicala is the President, CEO and co-founder of Project Assistants, Inc., a Project premier partner and leading provider of consulting services, custom development, education, and products for enterprise project management solutions. Mr. Cicala is also a popular speaker and writer on Project Portfolio management and the use of Project as an enterprise project management technology platform, and he serves on the Project Product Advisory Council.

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