Understanding Scheduling in Microsoft Project 2002

Applies to
Microsoft Project 2002

As a project manager, your schedule is one of the most critical tools used to manage a project to completion. Microsoft Project helps you schedule a project from the information you enter about the overall project, the individual work items (called tasks) required to complete the project, and, if necessary, the resources (the people, equipment, and materials) needed to complete those tasks. If anything about your project changes after you create your schedule, you can update the tasks or resources and Microsoft Project adjusts the schedule for you.

You can learn more about the ways in which Microsoft Project adjusts your schedule based upon the changes you make by reviewing the answers to the following questions:

How does Microsoft Project schedule a project?

For each task, you enter durations, task dependencies, and constraints. Microsoft Project then calculates the start date and finish date for each task. You can enter resources in your project and then assign them to tasks to indicate which resource is responsible for completing each assignment, and to calculate how many machines are needed or how much material will be consumed.

If you enter resources, task schedules are further refined according to resource work, units, and working times entered on calendars. Other elements, such as lead time and lag time, task types, resource availability, and the driving resource can affect scheduling, so understanding the effects of these elements can help you maintain and adjust your schedule as needed.

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How does the project start date affect the schedule?

If you enter a start date for the project, by default, Microsoft Project schedules tasks to begin on the project's start date and calculates the project's finish date based on the last task to finish. As you enter more information about tasks, such as task dependencies, durations, and constraints, Microsoft Project adjusts the schedule to reflect more accurate dates for tasks.

Before and after creating a task dependency

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

2 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.

3 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.

Most projects should be scheduled from a start date. However, scheduling from the finish date can be useful for determining when a project must start if it must finish on a specific date. You can change various task and resource information to see what effect it has on the project's start date and determine the optimum project start date.

 Note   You must schedule a project from a start date or from a finish date; you cannot schedule from both start and finish dates. You pick which date you want to use (normally a start date), and Microsoft Project schedules the other date (normally a finish date) for you based on the information you enter into the project plan.

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What are the default settings for calculating the schedule?

Microsoft Project calculates the duration of tasks based on the definitions of the duration units on the Calendar tab of the Options dialog box (Tools menu). Just like a monthly calendar, when you use outside Microsoft Project, the year begins in January and each week begins on Sunday or Monday.

By default, when Microsoft Project calculates duration units, one day equals 8 hours, one week equals 40 hours, and one month equals 20 working days. If you enter start and finish dates for tasks and don't enter start and finish times, Microsoft Project uses 8:00 A.M. as the default start time and 5:00 P.M. as the default end time.

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How do constraints affect the schedule?

When you need to control the start or finish date of a task, you can add a constraint to the task. Flexible constraints work with task dependencies to make a task occur as soon or as late as the task dependency will allow. For example, a task with an As Soon As Possible (ASAP) constraint and a finish-to-start dependency will be scheduled as soon as the predecessor task finishes.

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.

Before and after applying a constraint

1 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.

2 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.

Inflexible constraints 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.

If a task constrained to a date has a predecessor that finishes too late for the successor to begin on the date specified in the constraint, negative slack can occur. To specify that constrained tasks with negative slack are scheduled according to their task dependencies rather than the constraint dates, on the Tools menu, click Options, and then click the Schedule tab. Clear the Tasks will always honor their constraint dates check box.

Constraint Type Scheduling Impact Description
As Soon As Possible (ASAP) Flexible With this constraint, Microsoft Project schedules the task as early as it can, given other scheduling parameters. No additional date restrictions are put on the task. This is the default constraint for newly created tasks in projects scheduled from the start date.
As Late As Possible (ALAP) Flexible With this constraint, Microsoft Project schedules the task as late as it can, given other scheduling parameters. No additional date restrictions are put on the task. This is the default constraint for newly created tasks in projects scheduled from the finish date.
Finish No Later Than (FNLT) Moderate This constraint indicates the latest possible date that you want this task to be completed. It can be scheduled to finish on or before the specified date. A predecessor won't be able to push a successor task with an FNLT constraint past the constraint date. For projects scheduled from the finish date, this constraint is applied when you enter a finish date for a task.
Start No Later Than (SNLT) Moderate This constraint indicates the latest possible date that you want this task to begin. The task can be scheduled to start on or before the specified date. A predecessor won't be able to push a successor task with an SNLT constraint past the constraint date. For projects scheduled from the finish date, this constraint is applied when you enter a start date for a task.
Finish No Earlier Than (FNET) Moderate This constraint indicates the earliest possible date that you want this task to be completed. The task cannot be scheduled to finish any time before the specified date. For projects scheduled from the start date, this constraint is applied when you enter a finish date for a task.
Start No Earlier Than (SNET) Moderate This constraint indicates the earliest possible date that you want this task to begin. The task cannot be scheduled to start any time before the specified date. For projects scheduled from the start date, this constraint is applied when you enter a start date for a task.
Must Start On (MSO) Inflexible This constraint indicates the exact date on which a task must be scheduled to begin. Other scheduling parameters such as task dependencies, lead or lag time, resource leveling, and delay can't affect scheduling the task unless this requirement is met.
Must Finish On (MFO) Inflexible This constraint indicates the exact date on which a task must be scheduled to be completed. Other scheduling parameters such as task dependencies, lead or lag time, resource leveling, and delay can't affect scheduling the task unless this requirement is met.

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How do deadline dates affect the schedule?

Deadline dates don't usually affect task scheduling. Deadline dates 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, Microsoft Project displays a task indicator notifying you that the task missed its deadline.

Deadline dates can affect total slack on tasks. If you enter a deadline date before the end of the task's total slack, total slack will be recalculated 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 nonworking time, such as weekends and holidays, for the project. They are used to determine resource availability, how resources assigned to tasks are scheduled, and how tasks themselves are scheduled. Project and task calendars are used in scheduling tasks, and if resources are assigned to tasks, resource calendars are used as well.

The calendars referred to in Microsoft Project are:

  • Base calendars     These are the foundations for the other types of calendars. You can also choose a base calendar to be the project calendar, and you can apply a base calendar to tasks as a task calendar, or as the default hours for a resource calendar. Microsoft Project provides three base calendars, the Standard, 24-Hours, and Night Shift calendars. Resource calendars are based on the Standard calendar by default. You can customize your own base calendar using any of the base calendars provided.
  • Project calendars     These set the standard working and nonworking times for the project as a whole. If resource calendars or task calendars are not used, tasks are scheduled during the working time on the project calendar, by default.
  • Resource calendars     These are based on the Standard calendar by default. You can change working time or nonworking time for specific resources or a set of resources, ensuring that resources are scheduled only when they're available for work. If you have changed working or nonworking time on a resource calendar and the resource is assigned to a task, the task is scheduled during the working time on the resource calendar.
  • Task calendars     These can be used to define working times for tasks outside the working times in the project calendar. When a task calendar is assigned to a task and the resource assigned to the task has different working times in its resource calendar, the task is scheduled for the intersection of the two calendars' working times. But you can set a task option to ignore resource calendars and schedule the task through the resource's nonworking time.

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How do resource assignments drive the schedule?

If you don't assign resources to tasks in your project, Microsoft Project calculates the schedule using task 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 responsible for completing the task. More than one resource can be assigned to a task. Both work resources and material resources can be assigned to tasks. Unlike work resources, assigning material 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, this intersection of the task and resource is the assignment. The scheduling of this task depends on Sean's resource calendar and assignment units, in addition to task information such as duration, dependencies, constraints, and calendars.

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

  • The amount of work or overtime work the resource is assigned to do, and how that work is distributed over time. Work distribution over time can also be affected by work contours.
  • The number of assignment units for the resource, that is, part-time, full-time, or multiple, on the task.
  • The task type, which affects how a schedule changes if you revise the existing assignment. The three task types are fixed unit, fixed duration, and fixed work.
  • Whether the task is effort-driven. If a task is effort-driven, as resources are added or removed on the assignment, the work remains constant for the task and is redistributed among the resources. For fixed-unit tasks, for example, one result is that if more resources are assigned, it will take less of a duration to complete the task.
  • Resource calendars. Microsoft Project schedules assigned resources based on working and nonworking times indicated on their resource calendars.

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What is the difference between planned, scheduled, and actual dates?

After you have entered all pertinent project information and are ready to begin the project, you can set a baseline and begin to track actual progress on tasks. When you enter an actual start date or actual finish date for a task, Microsoft Project updates the scheduled dates for that task in the Start and Finish fields. The baseline start and finish dates, however, are not affected by changes you make to the actual or scheduled dates.

When you update or enter progress on tasks, Microsoft Project calculates the difference between a task's baseline start dates and the scheduled start dates, and shows that difference in the Start Variance field. The variance is also calculated between the baseline finish dates and scheduled finish dates, shown in the Finish Variance field. When tasks are completed, the variance is calculated between the baseline start and finish dates and the actual start and finish dates.

If changes you've made to tasks affect their start or finish dates or their successors' start or finish dates, the new dates become scheduled start and finish dates. Scheduled start and finish dates show the current status of the project as a result of the changes you made.

You can also compare baseline, scheduled, and actual dates to dates in an interim plan if you want to see incremental progress on your project.