Total Slack

Your Slack workspace. A Slack workspace is made up of channels, where team members can communicate and work together.Keep reading to learn about four key features of Slack. 💬 Organized conversations. In Slack, team members send messages and share files in channels. Create channels for teams, projects, office locations, or anything else that’s relevant to your organization. Total slack The total slack is equal to the allowable delay of an activity i without causing a violation of the project deadline and can be calculated as LF i - EF i = LS i - ES i with ES i and EF i the earliest start and earliest finish of each activity i using forward calculations (i.e. Obtained from the ESS). Free Slack is the number of days that an activity can have before it starts delaying the next (successor) activity. Total Slack is the number of days that activity can have before it starts delaying the whole project. Let's assume that we have a project that consists of 4 tasks.

Total Slack Equation
Total Slack Ms Project
Total Slack

Learn Total Slack definition in project management with explanation to study “What is Total Slack”. Study total slack explanation with project management terms to review project management course for online MBA programs.

Total Slack Definitions

Total slack is is the difference between the calculated earliest finish time of the very last activity and the project's required completion time.
Successful Project Management by Jack Gido and James P. Clements
Total slack/total float is measured by the amount of time that a schedule activity can be delayed or extended from its early start date without delaying the project finish date or violating a schedule constraint.
A Guide to the Project Management Body of Knowledge by Project Management Institute
Total Slack can be defined as the difference between the latest allowable date and the earliest expected date.
Project Management: A Systems Approach to Planning, Scheduling, and Controlling by Harold Kerzner?

Total Slack Explanation

Sometimes an activity takes more time than the estimated duration. This time must be calculated in order to know if this delaying has any effect on the total project completion time. The difference between the earliest finish time of the last activity of the project and the required project completion time is called total slack. Total slack is also called float. Total slack can be positive or negative. A positive slack shows the maximum amount to time the activity can be delayed without affecting the project completion time. Negative slack time shows the amount of time the activities must be hurried in order to complete the project on time. A zero slack shows that the project will be completed exactly on time and no activity can be postpones or delayed not just even for a little bit.

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Total Slack Equation

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The determination of the critical path of a project requires forward and backward calculation steps (see “Scheduling projects: How to determine the critical path using activity slack calculations?”) and allows for the calculation of three different slack values. In this article, the following three definitions of activity slack will be discussed:

Total slack
Safety slack
Free slack

The three definitions will be illustrated on the example project depicted in figure 1. The values for the activity slack depend on the predefined project deadline and therefore, a distinction is made between the following two situations:

When no project buffer is added, D(ESS) = D(LSS): the project deadline is equal to the minimal critical path length
When a project buffer is added on top of the minimum critical path length, D(ESS) ≠ D(LSS): the project deadline is set to a later time instance than the minimal critical path length

with D(ESS) the project duration of an earliest start schedule (every activity as soon as possible) and D(LSS) the project duration of a latest start schedule (every activity as late as possible).

Figure 2 displays an ESS with a critical path length of 15 time periods and hence D(ESS) = 15. Figure 3 displays a LSS with a predefined project deadline of 19 time periods (D(LSS) = 19)). In this case, a project buffer of 4 time units is added to the project as an extra safety time margin.

?Figure 2. An earliest start schedule for the example project (critical path length = 15 time units)

?Figure 3. A latest start schedule for the example project (within a predefined project deadline of 19 time periods)

Slack calculations

In table 1, the earliest and latest start and finish times are displayed for the project network of figure 1 under two scenarios. In a first scenario, the predefined project deadline is equal to 15, i.e. D(ESS) = D(LSS) = 15 while in a second scenario, the project deadline is set equal to 19 time periods and hence D(ESS) = 15 ≠ D(LSS) = 19. The values in the table will be used to define and calculate the three different slack values.

Table 1. The earliest/latest start/finish times for two different project schedules

D(ESS) = D(LSS) = 15				D(ESS) = 15 ≠ D(LSS) = 19
ES	LS	EF	LF	ES	LS	EF	LF
A	0	0	5	5	0	4	5	9
B	5	5	8	8	5	9	8	12
C	5	7	6	8	5	11	6	12
D	8	9	10	11	8	13	10	15
E	8	8	11	11	8	12	11	15
F	11	11	15	15	11	15	15	19

Total slack

The total slack is equal to the allowable delay of an activity i without causing a violation of the project deadline and can be calculated as

LF_i - EF_i = LS_i - ES_i

with ES_i and EF_i the earliest start and earliest finish of each activity i using forward calculations (i.e. obtained from the ESS). Similarly, the LS_i and LF_i are used to denote the latest start and latest finish of activity i using backward calculations (i.e. obtained from the LSS).

The total slack is given in table 2 for a project deadline of 15 and 19. Note that in case D(ESS) = D(LSS), the activities with a total slack value equal to zero belong to the critical path.

Safety slack

The safety slack is equal to the allowable delay of an activity i when all predecessors finish as late as possible and can be calculated as

LS_i - max_{all predecessors j}(LF_j)

with LF_j the latest finish time of an activity j that precedes activity i. The safety slack values are given in table 2 for the two project deadlines.

Free slack

Total Slack Ms Project

The free slack of an activity i is equal to the allowable delay of an activity that has no effect on the earliest start ES of a successor activity and can be calculated as

min_{all successors j}(ES_j) - EF_i

with ES_j the earliest start time of an activity j that succeeds activity i. The free slack values are given in table 2 for the two project deadlines.

Table 2. The three slack versions for two different project schedules

Total Slack

D(ESS) = D(LSS) = 15			D(ESS) = 15 ≠ D(LSS) = 19
Total slack	Safety slack	Free slack	Total slack	Safety slack	Free slack
A	0	0	0	4	n.a.	0
B	0	0	0	4	0	0
C	2	2	2	6	2	2
D	1	1	1	5	1	1
E	0	0	0	4	0	0
F	0	0	0	4	0	n.a.

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