Page 292 - The Handbook for Quality Management a Complete Guide to Operational Excellence

278 C o n t i n u o u s I m p r o v e m e n t D e f i n e S t a g e 279

The slack time for an activity x,y is the difference between
1. The latest time of event y
2. The earliest time of event x plus the estimated activity time

Slack time for an activity is the difference between the latest and earli­
est times for a given activity. Thus, assuming everything else remains on
schedule, the slack for an activity indicates how much delay in reaching
the activity can be tolerated without delaying the project completion.
Slack times for the activities in the house construction project are shown
in Table 13.6.
Events and activities with slack times of zero are said to lie on the
critical path for the project. Conversely, a critical path for a project is
defined as a path through the network such that the activities on this path
have zero slack. All activities and events having zero slack must lie on a
critical path, but no others can. Figure 13.8 shows the activities on the
critical path for the housing construction project as thick lines.

Control and Prevention of Schedule Slippage
Project managers can use the network and the information obtained from
the network analysis in a variety of ways to help them manage their proj­
ects. One way is, of course, to pay close attention to the activities that lie
on the critical path. Any delay in these activities will result in a delay for

Activity Slack
Excavate (1,2) 2 - (0 + 2) = 0
Foundation (2,3) 6 - (2 + 4) = 0
Rough wall (3,4) 16 - (6 + 10) = 0
Rough exterior plumbing (4,5) 20 - (16 + 4) = 0
Roof (4,6) 26 - (16 + 6) = 4
Rough electrical work (4,7) 25 - (16 + 7) = 2
Rough interior plumbing (5,7) 25 - (20 + 5) = 0
Exterior siding (6,8) 33 - (22 + 7) = 4
Wall board (7,9) 33 - (25 + 8) = 0
Exterior painting (8,10) 42 - (29 + 9) = 4
Flooring (9,11) 38 - (33 + 4) = 1
Interior painting (9,12) 38 - (33 + 5) = 0
Exterior fixtures (10,13) 44 - (38 + 2) = 4
Interior fixtures (12,13) 44 - (38 + 6) = 0

Table 13.6 Calculation of Slack times for House Construction Activities

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