Slack (slack: The amount of time that a task can slip before it affects another task or the project's finish date. Free slack is how much a task can slip before it delays another task. Total slack is how much a task can slip before it delays the project.) is the amount of time that a task can slip before it affects another task or the project's finish date. Use the Early Start, Early Finish, Late Start, Late Finish, Start Slack, and Finish Slack fields to analyze project schedule delays and find available slack in your schedule.
Note Slack is also generally called "float" in project management circles.
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Learn about slack (float)
The amount of slack in your schedule indicates how long tasks (task: An activity that has a beginning and an end. Project plans are made up of tasks.) can be delayed (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.) before other tasks or the project finish date (finish date: The date that a task is scheduled to be completed. This date is based on the task's start date, duration, calendars, predecessor dates, task dependencies, and constraints.) are affected. If you know where slack occurs in your schedule, you can move tasks when certain phases of the schedule have no slack and other phases have too much.
Free slack (free slack: The amount of time that a task can be delayed without delaying its successor tasks. For a task without successors, free slack is the amount of time that the task can be delayed without delaying the finish date of the project.) is the amount of time a task can be delayed before its successor (successor: A task that cannot start or finish until another task starts or finishes.) task is delayed. Use the Free Slack field to determine whether a task has any time available for a delay. This can be useful if a resource needs more time on a task, or if you want to assign a resource to another task. You can also use the Free Slack field to determine how to recover a schedule that is slipping.
Total slack (total slack: The amount of time that the finish date of a task can be delayed without delaying the finish date of the project.) is the amount of time a task can be delayed before the project finish date is delayed. Total slack can be positive or negative. If total slack is a positive number, it indicates the amount of time that the task can be delayed without delaying the project finish date. If total slack is a negative number, it indicates the amount of time that must be saved so that the project finish date is not delayed. Negative slack indicates that there is not enough time scheduled for the task and is usually caused by constraint (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].) dates.
By default and by definition, a task with 0 slack is considered a critical task (critical task: A task that must be completed on schedule for the project to finish on time. If a critical task is delayed, the project completion date might also be delayed. A series of critical tasks makes up a project's critical path.). If a critical task is delayed, the project finish date is also delayed.
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Use the Detail Gantt view to find slack (float)
- On the View menu, click More Views.
- In the Views list, click Detail Gantt, and then click Apply.
- On the View menu, point to Table, and then click Schedule.
In the chart portion of the view, slack appears as thin bars to the right of tasks, with slack values adjoining the regular Gantt bars.
Press TAB to move to the Free Slack and Total Slack fields if they are not visible in the sheet portion of the view.
- If you know where slack occurs in your schedule, you can move tasks when certain phases of the schedule have no slack and other phases have too much.
- Slack values may also indicate a schedule inconsistency. For example, a negative slack value occurs when one task has a finish-to-start dependency with a successor task, but the successor task has a Must Start On constraint that is earlier than the end of the first task. Negative slack can also occur when a task is scheduled to finish after its deadline date.
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Use task fields to find slack (float)
Add the Early Start, Early Finish, Late Start, and Late Finish fields (field: A location in a sheet, form, or chart that contains a specific kind of information about a task, resource, or assignment. For example, in a sheet, each column is a field. In a form, a field is a named box or a place in a column.) to a task view to analyze delays and find any available slack time in your schedule.
Early Start and Early Finish fields
When you first create a task, its early start and early finish (early finish date: The earliest date that a task could possibly finish, based on early finish dates of predecessor and successor tasks, other constraints, and any leveling delay.) dates are the same as the scheduled start and finish dates. As you link the task to predecessor (predecessor: A task that must start or finish before another task can start or finish.) and successor tasks and apply any date constraints, the early start and early finish dates are calculated as the earliest possible dates this task could start and finish if all predecessors and successors also start and finish on their respective early start and early finish dates. If there is a leveling delay (leveling delay: The amount of time that an assignment or task is to be delayed from the original scheduled start date as a result of resource leveling or of manually entering a leveling delay.) on the task, this delay is also figured into the early start and finish dates. These calculations are based on a fixed duration (fixed-duration task: A task in which the duration is a fixed value and any changes to the work or the assigned units [that is, resources] don't affect the task's duration. This is calculated as follows: Duration x Units = Work.).
Late Start and Late Finish fields
When you first create a task, its late finish (late finish date: The latest date that a task can finish without delaying the finish of the project. It is based on the task's late start date, as well as the late start and late finish dates of predecessor and successor tasks, and other constraints.) date is the same as the project finish date, and its late start date is calculated as the project finish date minus the task 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.). As you link the task to predecessors and successors and apply any other constraints, Project calculates the late start date as the last possible date this task could be started if all predecessor and successor tasks also start and finish on their late start and late finish dates.
Likewise, the late finish date is calculated as the latest possible date this task could finish if all predecessor and successor tasks also start and finish on their late start and late finish dates. If there is a leveling delay on successor tasks, this delay is also figured into the date in the Late Start and Late Finish fields. These calculations are based on a fixed duration.
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Use deadlines to create slack (float)
In Microsoft Office Project 2007, you can set deadlines (deadline: A target date indicating when you want a task to be completed. If the deadline date passes and the task is not completed, Project displays an indicator.) for a task. Deadlines affect the calculation of late finish dates, the backward pass (backward pass: Calculation of late finish and late start dates for project activities, determined by working backward from the project's finish date.), and the total slack of a task.
The deadline usually sets the late finish for a task and therefore affects the backward pass. The following cases describe the exceptions to this rule:
Other task constraints on a task with a deadline might also affect late start or early start dates. For example, a task's late start date plus its duration might not necessarily equal its late finish date.
Tasks with As Late As Possible (ALAP) constraints will typically finish on the deadline date, but predecessor tasks might push a task beyond its deadline. In that case, Office Project 2007 displays an indicator (indicators: Small icons representing information for a task or resource that are displayed in the Indicators field. The Indicators field is located to the right of the ID field and appears in a number of tables.) that the deadline is violated.
When you schedule a project from the finish date, deadlines also affect the late finish of the task. Even though a task is usually scheduled to finish on the deadline date, a predecessor with an MSO constraint, for example, can push a task beyond its deadline and Office Project 2007 displays an indicator that the deadline is violated.
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Review free slack (float) for leveling projects
Add the Start Slack and Finish Slack fields to any task view when you want to review how free slack was calculated.
The Start Slack field contains the duration representing the difference between the early start and late start dates. The Finish Slack field contains the duration representing the difference between the early finish and late finish dates. The smaller of the start slack and finish slack amounts determines the amount of free slack available, that is, the amount of time a task can be delayed without affecting the start date of a successor task.
If a task has an actual (actual: Information that shows what has actually occurred. For example, the actual start date for a task is the day that the task actually started.) start date and a deadline, the start slack is 0 and the finish slack is the difference between the task's finish date and deadline date.
Start slack is useful when leveling (leveling: Resolving resource conflicts or overallocations by delaying or splitting certain tasks. When Project levels a resource, its selected assignments are distributed and rescheduled.) projects scheduled from the finish date. Finish slack is useful when leveling projects scheduled from the start date.
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