6 factors that affect how Project calculates time

Applies to
Microsoft Office Project 2003
In this article


Introduction

Scheduling tasks (task: An activity that has a beginning and an end. Project plans are made up of tasks.) in your project plan (plan: A schedule of task start and finish dates and resource and cost data. A baseline plan is the original plan that you save and use to monitor progress. An interim plan is a set of dates you save during the project to compare to other interim plans.) to reflect the reality of your situation, as well as your customer requirements, can be a challenge. When you master the six factors that affect the calculation of time in Project 2003, you will be well on your way to having an optimized project plan.

The six key factors that drive the calculation of time — that is, of dates and duration (duration: The total span of active working time that is required to complete a task. This is generally the amount of working time from the start to finish of a task, as defined by the project and resource calendar.) — in Project are:

  • Project start date
  • Task durations
  • Task dependencies
  • Project calendars
  • Task constraints and deadlines
  • Resource assignments and task types

When you understand how these six aspects of scheduling affect your project plan, you will have the necessary framework to develop and maintain your project plans. You will also know how to troubleshoot and eliminate any problems that occur when you optimize your plan for time, which is one side of the project triangle (project triangle: The interrelationship of time, money, and scope. If you adjust any one of these elements, the other two are affected. For example, if you adjust the project plan to shorten the schedule, you might increase costs and decrease scope.). This article gives tips and techniques for using the six key factors that affect the calculation of a task's start date, finish date, and duration.

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Establish the project start date

The first scheduling factor that Project considers for a task is the project start date (start date: The date when a task is scheduled to begin. This date is based on the duration, calendars, and constraints of predecessor and successor tasks. A task's start date is also based on its own calendars and constraints.), as set in the Project Information dialog box. The date on which you create the project plan becomes the default project start date. However, you can change this date to reflect the plan that you are creating.

The project start date is important because all of the tasks will start on the project start date until you add task dependencies or 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].).

Even after you set up your project plan, you can run a what-if scenario (what-if scenario: A method that is used to forecast values based on information that you provide.) to see the effect of delaying (delay: The amount of time between the scheduled start of a task and the time when work should actually begin on the task; it is often used to resolve resource overallocations. There are two types of delay: assignment delay and leveling delay.) or moving up the start of a project. This can be a powerful method for quickly recalculating tasks.

To define your project start date:

  1. On the Project menu, click Project Information.
  2. In the Start date box, enter the project start date.

Dialog box with project start date

 Note    In the Project Information dialog box, you can use the Schedule from box to schedule from the start date or finish date. Although "backward scheduling" from the project finish date can be useful for initial planning that is based on an imposed deadline, your project plan will be easier to work with if you schedule from the start date. You can apply a deadline or a Must Finish On (MFO) constraint to a milestone (milestone: A reference point marking a major event in a project and used to monitor the project's progress. Any task with zero duration is automatically displayed as a milestone; you can also mark any other task of any duration as a milestone.) that signifies the end of the project. You can then adjust the project start date to create what-if scenarios and compare the results.

By default, the start date does not appear as a gridline (gridlines: The horizontal and vertical lines that appear in many Project views. You can change the patterns and colors of gridlines, and you can control the interval at which different patterns and colors occur.) in the Gantt Chart view. You might want to display both your project start date and the current date in the Gantt Chart view to easily visualize where you are in the project now compared with where you were when the project started.

To create a blue gridline to represent the project start date and a red gridline to represent today's date:

  1. On the Format menu, click Gridlines.
  2. Under Line to change, click Project Start.
  3. Under Normal, select a solid line in the Type box.
  4. Under Normal, select Blue in the Color box.
  5. Under Line to change, click Current Date.
  6. Under Normal, select a solid line in the Type box.
  7. Under Normal, select Red in the Color box.

Gridlines dialog box

The resulting gridlines in the chart area of the Gantt Chart view show where you are today in relation to where you were on the project start date.

Gantt Chart with gridlines

 Note   If your start date and today's date are identical, you will see only the line that represents the project start date.

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Enter task durations

Task durations have a direct impact on the calculation of start dates and finish dates. You can enter task durations directly into the Duration field in any table or dialog box where the field is displayed. The most typical method is to enter a task's duration in the Duration field of the Gantt Chart view by using the Entry table.

The following table shows abbreviations that you can use when you enter durations.

For Enter Example
Minutes m 30m
Hours h 2h
Days d 4d
Weeks w 6w
Months mo 1mo

Remember that duration is based on working days as defined in your project calendar (project calendar: The base calendar used by a project.). According to the default Standard calendar, 8 hours = 1 day, 5 days = 1 week, and 20 days = 1 month. For example, suppose that you want to specify that a task will take 1 month to complete. If you enter 30 days in the Duration field, Project converts that to 6 weeks rather than to 1 month.

To review these duration conversions:

  1. On the Tools menu, click Options.
  2. Click the Calendar tab.
  3. Review the Hours per day, Hours per week, and Days per month boxes.

Calendar options fields

 Note   Be careful not to change these settings without also changing the corresponding project calendar settings and working times.

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Determine task dependencies

One of the most critical steps in project scheduling is to establish a network for your tasks by setting task dependencies. With dependencies, you reflect the order in which tasks should be done, as well as the relationship between sequential tasks. If one task cannot start until another task finishes, you can link (linking: In a project, establishing a dependency between tasks. Linking tasks defines a dependency between their start and finish dates. In OLE, establishing a connection between programs so that data in one document is updated when it changes in another.) the two tasks by using the common finish-to-start (FS) dependency. If a task can start as soon as another task starts, you can link them by using the start-to-start (SS) dependency. You can also set finish-to-finish (FF) dependencies and start-to-finish (SF) dependencies.

When the task dependencies are set and the network is established, you can easily identify the critical path (critical path: The series of tasks that must be completed on schedule for a project to finish on schedule. Each task on the critical path is a critical task.) and understand the driving factors to the project end date. You can also easily make changes to one task and immediately see the ripple effects on the rest of the plan.

Most projects have several clusters of linked tasks. The first task in each cluster (or "path") begins at the project start date. Each subsequent task is linked to the predecessor (the one that comes before it) and the successor (the one that comes after it). A task can have multiple predecessors (predecessor: A task that must start or finish before another task can start or finish.) and successors (successor: A task that cannot start or finish until another task starts or finishes.).

Consider creating a project-complete milestone in your plan to link other tasks or milestones. This can ensure that you are alerted early enough when the project finish date is in danger of being pushed out.

To set a finish-to-start dependency by using the Link Tasks button:

  1. In a task sheet (such as the Gantt Chart), select the predecessor task first and the successor task next.
  2. On the Standard toolbar, click Link Tasks Button image.

Gantt Chart with task selected

 Note   You can select several tasks at once, and then click Link Tasks on the Standard toolbar. This creates a finish-to-start relationship between all the selected tasks.

To set dependencies by using the Task Entry view:

  1. In the Gantt Chart view, click Split on the Window menu.

The Task Entry combination view appears, showing the Gantt Chart in the upper pane and the Task Form in the lower pane.

  1. In the Gantt Chart view, select the task to which you want to add a predecessor.
  2. In the Task Form, click below the Predecessor Name heading, and then select the task that you want to link as the predecessor to the task that is selected in the Gantt Chart view.
  3. In the Type field, select the dependency type: FF, FS, SF, or SS.
  4. If necessary, use the Lag field to create lead times or lag times between tasks.

Specify lead time (lead time: An overlap between tasks that have a dependency. For example, if a task can start when its predecessor is half-finished, you can specify a finish-to-start dependency with a lead time of 50 percent for its successor [has a negative lag value].) by using a negative duration or percentage amount, indicating how early the current task can start or finish in relation to the predecessor. Specify lag time (lag time: A delay between tasks that have a dependency. For example, if you need a two-day delay between the finish of one task and the start of another, you can establish a finish-to-start dependency and specify a two-day lag time [has a positive value].) by using a positive duration or percentage amount, indicating how much time should pass after the start or finish of the predecessor before the current task can start or finish.

  1. To return to the full Gantt Chart view, click Remove Split on the Window menu.

Tip     You can also select a task, open the Task Information dialog box, and then set the predecessors by using the Predecessors tab. The Type and Lag fields are available on this tab as well.

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Define and apply calendars

Project also uses calendars to calculate the start dates and end dates of tasks. Three base calendars (base calendar: A calendar that can be used as a project and task calendar that specifies default working and nonworking time for a set of resources. Differs from a resource calendar, which specifies working and nonworking time for an individual resource.) are included with Project: the Standard calendar, the 24-Hours calendar, and the Night Shift calendar. By default, the Standard calendar is applied to the entire project, reflecting a working time of Monday through Friday, 8:00 A.M. to 5:00 P.M., with one hour off for lunch.

To change the base calendar that is used for your project:

  1. On the Project menu, click Project Information.
  2. In the Calendar box, select the calendar that you want to apply to the project.

You can modify any of the built-in calendars to suit your project team's working time.

To modify a built-in calendar:

  1. On the Tools menu, click Change Working Time.
  2. In the For box, select the built-in calendar that you want to change.
  3. In the calendar grid, select the day of the week whose working time you want to change.

For example, to change the working time for all Wednesdays, click W in the calendar grid.

  1. To change the working time for the day, modify the times in the From and To boxes.
  2. To change a working day to a nonworking (nonworking time: Hours or days designated in a resource or project calendar when Project should not schedule tasks because work is not done. Nonworking time can include lunch breaks, weekends, and holidays, for example.) day, select the day of the week and then, under Set selected date(s) to, click the Nonworking time option.
  3. To change a nonworking day to a working day, select the day of the week and then, under Set selected date(s) to, click the Nondefault working time option.

You can also use project calendars to simulate a what-if scenario. For example, suppose that you are on a tight schedule and you are wondering whether having your team work on Saturdays would help meet the desired project finish date.

To create a new "Saturdays" base calendar:

  1. On the Tools menu, click Change Working Time.
  2. Click New.
  3. In the Name field, type Saturday Calendar and then click OK.
  4. In the calendar grid, click S to select all Saturdays.
  5. Under Set selected date(s) to, click the Nondefault working time option.
  6. In the From and To boxes, make any necessary changes to the working times.

Change Working Time dialog box

You can now select your new Saturday Calendar as the project calendar in the Project Information dialog box. Your project plan will be rescheduled according to this new calendar, and you can see whether it favorably affects the project's timeline.

 Notes 

  • You can create a new base calendar if your team (or a group of resources within the team) has entirely different working times from those reflected by any of the three built-in base calendars.
  • To apply a calendar that is different from the project calendar to a specific resource, select the resource in any resource view. Open the Resource Information dialog box, and then click the Working Time tab.
  • To apply a calendar that is different from the project calendar to a specific task, select the task. Open the Task Information dialog box, and then click the Advanced tab. Select the calendar in the Calendar box.

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Change task constraints

Project schedules all tasks by using the As Soon As Possible (ASAP) constraint unless you explicitly apply a date constraint, such as Start No Earlier Than (SNET) or Must Finish On (MFO).

One of the most common mistakes made in scheduling with Project is to type the dates directly into the Start or Finish date field. When you type a date in the Start field in the table portion of the Gantt Chart view, this creates a Start No Earlier Than (SNET) constraint. When you type a date in the Finish field, this creates a Finish No Earlier Than (FNET) constraint.

Having such date constraints in your schedule can cause frustration, especially when you later try to make even the most minor adjustment, respond to a deadline change, or run a what-if scenario. Most likely you will have to manually reenter the affected task constraint dates. In addition, date constraints can skew the critical path calculations and limit your ability to manage the project life cycle. A schedule that is based on date constraints restricts the flexibility that Project has to calculate the best task dates. In turn, this hinders your ability to use Project as an effective management tool.

Instead of using date constraints to drive your schedule, it is best to set up a network of task dependencies that drives the start date and finish date for most tasks. Use a date constraint only for special tasks that have a "hard-coded" date, such as a trade show or a contract deadline.

The following table lists all constraints. As Soon As Possible (ASAP) is the default constraint for a project that is scheduled from the project start date. As Late As Possible (ALAP) is the default constraint for a project that is scheduled from the project finish date. These two constraints provide the most scheduling flexibility. All other constraints are fixed on a specific date.

Constraint Description
As Late As Possible (ALAP) Schedules the task as late as it can occur in the schedule without delaying subsequent tasks. Do not enter a constraint date with this constraint.
As Soon As Possible (ASAP) Schedules the task to begin as early as it can occur. This is the default constraint for tasks. Do not enter a constraint date with this constraint.
Finish No Earlier Than (FNET) Schedules the task to finish on or after the constraint date. Use it to ensure that a task does not finish before a certain date.
Finish No Later Than (FNLT) Schedules the task to finish on or before the constraint date. Use it to ensure that a task does not finish after a certain date.
Must Finish On (MFO) Schedules the task to finish on the constraint date. Sets the early, scheduled, and late finish dates to the date that you type and anchors the task in the schedule.
Must Start On (MSO) Schedules the task to start on the constraint date. Sets the early, scheduled, and late start dates to the date that you type and anchors the task in the schedule.
Start No Earlier Than (SNET) Schedules the task to start on or after the constraint date. Use it to ensure that a task does not start before a specified date.
Start No Later Than (SNLT) Schedules the task to start on or before the constraint date. Use it to ensure that a task does not start after a specified date.

To change the constraint:

  1. In a task sheet (such as the Gantt Chart), select the task whose constraint you want to change.
  2. On the Project menu, click Task Information.
  3. Click the Advanced tab and then, in the Constraint type list, click the appropriate constraint.
  4. If you select a date constraint (any constraint other than As Soon As Possible or As Late As Possible), enter the date in the Constraint date box.

Task Information dialog box

A good alternative to setting a date constraint that affects the project schedule is to apply a task deadline. A deadline provides a reminder of date goals without constraining the schedule itself. A flag appears when a task deadline has passed without the task being marked as complete.

To set a task deadline:

  1. In a task sheet (such as the Gantt Chart), select the task to which you want to apply a deadline.
  2. On the Project menu, click Task Information.
  3. Click the Advanced tab and then, in the Deadline box, enter the deadline.

Dialog box with deadline entered

 Notes 

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Assign resources with task types

Until you assign resources, task scheduling depends on the project start and finish dates, task durations, task dependencies, project calendar, and constraints. After you assign resources to tasks in the project, Project takes more resource-related factors into consideration. The most significant factor related to assigned resources is the task type.

The task type indicates whether the task is driven by the number of resources assigned to the task (the default), the amount of work (for example, hours or the level of effort) assigned to the resources on the task, or the task duration. Project calculates these three variables — units, work, or duration — each time a resource is assigned to a task. These variables are tied together in a formula based on Work = Duration * Units.

If you change one variable, one of the other two variables will be recalculated, depending on what task type has been selected for that task.

The task type that you choose for a task depends on the scheduling technique that you want to be applied to that task or to most of the project. There are two main approaches to scheduling. One is duration-based and the other is effort-based:

  • Duration-driven scheduling is best used for tasks or projects that are driven by dates and deadlines and that will not be affected by the quantity of assigned resources. A good example of this is a trade show. No matter how many resources you assign to the trade show task, the duration is always the same.
  • Effort-driven tasks are those for which the duration should be recalculated based on the quantity of assigned resources. The more resources that are assigned to a task, the less time it takes to complete the task. Likewise, if resources are removed from an effort-driven task, the duration increases. A good example is the so-called "envelope stuffing" task.

To change the task type for individual tasks:

  1. Select the tasks whose task type you want to change.
  2. On the Project menu, click Task Information, and then click the Advanced tab.
  3. In the Task type box, click the task type (Fixed Work, Fixed Duration, or Fixed Units) that you want for the selected tasks.
  4. Select or clear the Effort driven check box.

By default, the Fixed Work task type is effort driven.

Dialog box with Task type circled

 Note   For fixed-duration tasks, you might consider clearing the Effort driven check box. If Effort driven is selected for fixed-duration tasks, work remains constant within the fixed-duration time period and is redistributed each time there is a resource change that is based on the original work calculation.

Fixed Units is the default task type in Project. If you expect most of your tasks to be either Fixed Duration or Fixed Work, you probably will want to change the default task type when you first create your project and before you enter tasks. Otherwise, you might have to rework the plan and change each task type setting. Changing the default task type affects only new tasks, not existing tasks.

To change the default task type for a project plan:

  1. On the Tools menu, click Options.
  2. Click the Schedule tab.
  3. In the Default task type box, click the task type that you want for your default (for example, Fixed Duration or Fixed Work).

Dialog box with Default task type circled

As you continue to use Project, you will learn more of the ways in which the six factors for scheduling tasks interact with each other. By understanding how Project uses the project start date, durations, dependencies, calendars, constraints, and task types to adjust scheduling, you can use the power of Project to create a realistic schedule and manage the project throughout its life cycle.

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Applies to:
Project 2003