How Project schedules tasks: Behind the scenes

What do you want to learn?

• How does the project start date affect the schedule?

• How do task links affect the schedule?

• How do constraints on tasks affect the schedule?

• How do task types affect the schedule?

• How does effort-driven scheduling affect the schedule?

• How do manual and automatic scheduling affect the schedule?

• How does float (or slack) affect the schedule?

• How do deadline dates affect the schedule?

• How do calendars affect the schedule?

• How do resource assignments drive the schedule?

• Get more technical information about how Project schedules

How does the project start date affect the schedule?

When you add a new task to a schedule, it automatically is scheduled to start on the project’s start date. As tasks are added to the schedule and connected to other tasks, the start times of tasks will change, and the last task to finish determines the project’s finish date.

• To find the Project start date or to change it to another date, select Project, and then select Project Information.

Of course, there are going to be exceptions. For example, tasks don’t move around as other tasks move. Read below in this article to learn more about manually versus automatically scheduled task scheduling.

When you create a new project, you first enter the project's start date. When you schedule a project from the start time, all tasks start at the project start date unless you specify otherwise.

For both manually scheduled tasks and automatically scheduled tasks, with no task dependencies or constraints applied, the project's duration is the same as the duration of the longest task. In other words, the project finish date is the same as the longest task's finish date.

Task dependencies, such as the finish-to-start dependency between the first and second tasks (as shown here), can change the project's finish date.

Nearly all projects should be scheduled from a known start time. Even if you know the date that a project must be completed, scheduling from a start date gives you maximum flexibility.

However, you might want to schedule from a finish date when:

• You need to determine when a project must start so that it finishes on a specific required date.

• You are not sure when your project will begin (for example, you are receiving work from another source that could be delayed).

• Your project management methodology requires you to schedule from a finish date.

As you work with your project that is scheduled from a finish date, be aware of differences in the way that Project handles some actions:

• When you enter an automatically scheduled task, Project automatically assigns the As Late As Possible (ALAP) constraint to the finish date of the task. You should set other constraints only when necessary (Select a task, and then select Task > Information).

• If you drag a Gantt bar to change the finish date of a task, Project automatically assigns a Finish No Later Than (FNLT) constraint for an automatically scheduled task.

• If you change your project to schedule from a finish date and it was previously scheduled from a start date, you will remove all leveling delays and leveling splits from tasks and assignments that have been automatically scheduled. Tasks that are manually scheduled are not affected.

• If you use automatic leveling to reduce resource overallocations in your project, Project will add a leveling delay after a task rather than before a task (To check leveling settings, select the Resource tab, and then select Level Resource).

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How do task links affect the schedule?

Task links (dependencies) show the relationship in time of one task to another. Linking tasks creates task dependencies. The long chain of tasks determines the entire length and end of a project.

Project management tip As you link tasks in a project, a critical path of links is set up. This critical path determines the end of your project. Learn more about the critical path.

Note about manually scheduled tasks When you link a manually scheduled task to another task, Project respects the link type and places the manually scheduled task relative to the other task. For example, the successor task with a finish-to-start link is shown as beginning when the predecessor finishes. However, the successor task will only move when the link is created. If the predecessor later changes its finish date, the successor’s start date will remain unchanged.

However, you can configure Project so that a manually scheduled task does not move when it is linked to another task.

1. Select File > Options > Schedule.

2. Uncheck the Update manually scheduled tasks when editing links check box.

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How do constraints on tasks affect the schedule?

Constraints allow you to control the start or finish date of an automatically-scheduled task. There are three types of constraints:

• Flexible constraints do not have specific dates associated with them. Setting these constraints allows you to start tasks as early as possible or as late as possible, with the task ending before the project finish, given other constraints and task dependencies in the schedule.

• Semi-flexible constraints require an associated date that controls the earliest or latest start or finish date for a task. These constraints allow a task to finish at any time, as long as it meets the start or finish deadline.

• Inflexible constraints require an associated date that controls the start or finish date of the task. These constraints are useful when you need to make your schedule take into account external factors, such as the availability of equipment or resources, deadlines, contract milestones, and start and finish dates.

Project management tip In nearly all cases, use the ASAP constraint. This gives the scheduling engine the most flexibility in determining the ideal finish date for your project.

Here are two ways to instantly view the constraints on your tasks.

• To review or change a constraint, select a task, and then select Task > Information. Select the Advanced tab to see the Constraint type and Constraint date boxes.

• If the constraint is anything other than ASAP or ALAP, the constraint type will display its associated graphical indicator in the Indicator column in any sheet view, such as the Gantt chart.

Constraints with moderate scheduling flexibility will restrict a task from starting or finishing before or after a date you choose. For example, a task with a Start No Earlier Than (SNET) constraint for June 15 and a finish-to-start dependency to another task can begin June 15 if its predecessor is finished by June 15 (or later if its predecessor finishes after June 15), but it can't be scheduled before June 15. For example, this might be an appropriate use of constraints if you have a building permit that is only good for specific dates. In this case, the SNET or FNLT constraints might be used.

With the default finish-to-start task relationship and an ASAP constraint applied to these tasks, the successor task (the second one) is scheduled to begin as soon as the predecessor task (the first one) is scheduled to finish.

With a SNET constraint applied, the successor task cannot begin before the constraint date, even if (as shown here) the predecessor task is completed before the constraint date.

The following table lists the constraints provided in Project.

By default, all tasks in a project that is scheduled from the start date have the ASAP constraint applied. Likewise, by default, all tasks in a project that is scheduled from the finish date have the As Late As Possible (ALAP) constraint applied.

Inflexible constraints usually override any task dependencies and restrict a task to a date you choose. For example, a task with a Must Start On (MSO) constraint for September 30 and a finish-to-start dependency to another task will always be scheduled for September 30 no matter whether its predecessor finishes early or late. You can change this behavior. Select File > Options > Schedule, and then select the Tasks will always honor their constraint dates check box.

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How do task types affect the schedule?

Task types apply only to automatically scheduled tasks, and come in three flavors: fixed units, fixed work, and fixed duration. Project looks at a task’s type to determine how duration, work, and units will behave as Project schedules your project.

Each of the task types affects scheduling when you edit one of the three elements as follows.

• To change a task type, select the task, select Task > Information and go to the Advanced tab.

How does effort-driven scheduling affect the schedule?

For all tasks, after you assign a resource, the task is scheduled according to this formula (assuming tasks are the default fixed-units task type):

Duration = Work / Units

• For any task, you can choose which piece of the equation Project calculates by setting the task type. When you assign or remove people from a task, Project lengthens or shortens the duration of the task based on the number of resources that are assigned to it, but Project does not change the total work for the task. This is called effort-driven scheduling. This setting is usually turned off. To turn it on, select File > Options > Schedule > New tasks are effort driven check box.

Although effort-driven scheduling can work in most scenarios, you may want to change this behavior to more accurately reflect what happens on a particular task when resources are added or removed. For example, you may want to see the total work increase as you add more people to a particular task.

1. Select a task, select Task > Information, and then select the Advanced tab.

2. Uncheck the Effort driven check box.

When you work with effort-driven scheduling, keep the following in mind:

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How do manual and automatic scheduling affect the schedule?

Knowing the differences between manually scheduled tasks and automatically scheduled tasks is key to understanding how Project schedules your project.

Generally, manually scheduled tasks put you in control of the schedule. When you add a task to your schedule, it stays put. However, this isn’t always the best way to design a schedule, especially as projects get more complicated, where you might want to use automatically scheduled tasks in order to take advantage of Project’s powerful scheduling engine.

Take a look at the picture below. It shows the two different types of tasks, the first two manually scheduled, and the last two automatic.

Note that for the manually scheduled tasks, the duration is a text value as well as a number. Since the duration can be a text value, a start date hasn't been set automatically by Project, and the Gantt bar is only partially displayed to reflect the uncertainty of the task schedule at this point.

For the automatically scheduled task, the duration is a number value with a time unit appended, such as “12h” for twelve hours duration. By definition, automatically-scheduled tasks with valid durations, start dates, and finish dates—and therefore, bars—are automatically drawn by Project.

Now, it gets a little tricky sometimes with the information you provide for manually scheduled tasks. All that is needed for Project to draw bars on a manually scheduled task are three time values: duration, start date, and finish date. If you set two of these values for a manually scheduled task, the third value will be calculated by Project automatically, and the task will remain manually scheduled. So, look out for this kind of “unexpected” behavior from Project.

How does float (or slack) affect the schedule?

Float (also known as slack) helps you find those tasks that can budge without changing the end date of your project. You may want to view tasks that can currently slip without affecting the critical path (total slack) or those tasks that can slip before affecting the task that they are connected to (free slack).

Here are two ways to view float in your schedule.

• While in the Gantt chart, select Gantt Chart Format, then select the Slack check box. Float appears as a thin line attached to the end or beginning of Gantt bars.

• Use the Detail Gantt view. Select View > More Views, and then choose Detail Gantt. Select Apply. Now, back in the Gantt chart, select Tables > Schedule.

Here a couple things to keep in mind about float:

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How do deadline dates affect the schedule?

Deadline dates don't usually affect task scheduling. They are used to indicate a target date you don't want to miss, without requiring you to set a task constraint that could affect scheduling if predecessor tasks change. A task with a deadline is scheduled just like any other task, but when a task finishes after its deadline, Project displays a task indicator notifying you that the task missed its deadline.

To review or change a task deadline, select the task and select Task > Information. On the Advanced tab, use the Deadline box.

Deadline dates can affect the total slack on tasks. If you enter a deadline date before the end of the task's total slack, total slack will be recalculated by using the deadline date rather than the task's late finish date. The task becomes critical if the total slack reaches zero.

You can set deadlines for summary tasks as well as individual tasks. If the summary task's deadline conflicts with any of the subtasks, the deadline indicator signifies a missed deadline among the subtasks.

But deadline dates can affect how tasks are scheduled if you set a deadline date on a task with an As Late As Possible (ALAP) constraint. The task is scheduled to finish on the deadline date, though the task could still finish after its deadline if its predecessors slipped.

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How do calendars affect the schedule?

Calendars determine the standard working time and non-working time, such as weekends and holidays, for the project.

Project calendars are used to determine the resource availability, how resources that are assigned to tasks are scheduled, and how the tasks themselves are scheduled. Project and task calendars are used in scheduling the tasks, and if resources are assigned to tasks, resource calendars are used as well.

Calendars override each other’s settings in the following ways.

1. When you create a task, the project calendar schedules the task.

2. When you add a resource to a task, the resource calendar controls the scheduling of the task.

3. When you add a task calendar to a task and set the calendar setting to Scheduling ignores resource calendars in the Task Information box, the task calendar controls the scheduling and will ignore the schedule of any resources assigned to the task.

To work with calendars, select Project, then select Change Working Time.

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How do resource assignments drive the schedule?

If you don't assign resources to tasks in your project, Project calculates the schedule using durations, task dependencies, constraints, and project and task calendar information. If you do assign resources, the tasks are also scheduled according to resources' calendars and assignment units, providing for more accurate scheduling.

An assignment is the association of a specific task with a specific resource that is responsible for completing the task. More than one resource can be assigned to a task. Work resources, material resources, and cost resources can be assigned to tasks. Unlike work resources, assigning material resources or cost resources to a task does not affect task scheduling.

For example, in your project you have a task named Develop specifications. You also have an engineering resource, Sean. If you assign Sean to the Develop specifications task, the scheduling of this task depends on Sean's resource calendar and assignment units, in addition to task information such as duration, task dependencies, constraints, and calendars.

In addition to scheduling according to task information, after you assign resources to the tasks in your project, Project has additional resource and assignment information to use in calculating schedule information, including:

To assign resources to tasks, select the Resource tab, and then select Assign Resources.

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Get more technical information about how Project schedules

• Add holidays and vacation days

• Display the critical path to help bring in the end date of your project

• Set the start date (or finish date) for your project

• Top-down planning

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