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 3. FLOAT. "Float" is the amount of slack time within which the starting
point of an arrow that is not on the critical path can be moved without
disturbing the critical path. Arrows (steps) with zero float (no slack time)
are always on the critical path. The recognition of float allows the Master
Scheduler to (a) analyze the effect on noncritical steps when the duration
time of the critical path is reduced, and (b) either reduce the crew size
(spread the work) to utilize the available float to balance the work force or
vary the ultimate schedule starting date of the step within the float
limitations. An example of float is seen when step 7 in Figure A-7 is
scheduled. Since steps 5 and 7 are to be started and completed while steps 4
and 8 are being accomplished (see Figure A-6), an 11 hour float exists for
steps 5 and 7. Because it was considered desirable to start work on step 5 at
the beginning of the first week, along with steps 3, 4, and 6, step 7 can be
started anytime after 4 P.M. (assuming an 8 to 4:30 workday with 30 minutes
for lunch) on 4 May provided that the work is completed by 10:30 A.M. on
11 May. Because it would be uneconomical to have the craftsman start step 7
with only 30 minutes of working time left in the day, a later start is made.
Also, because enough float exists for this work, step 7 is scheduled to start
at 8:00 A.M. on 8 May, the beginning of the second week.
4. DECREASING JOB DURATION TIME. The initial critical path has been
developed through analysis of normal operating procedures and working hours.
Improving the critical path is the most important part of scheduling. The
improvement amount will depend upon the management objective for scheduling
this work, material procurement factors, available personnel, and the makeup
of the job itself.
a. Management Objectives. Normally, management schedules jobs to obtain
minimum effort and expense. However, management may dictate other objectives
such as:
(1) A directed completion date
(2) A directed starting date
(3) A minimum or shortest time plan involving overtime or other crash
methods
(4) Use of maximum numbers of personnel, with or without overtime
(5) Directed starting or completion dates for specific portions of
the job.
b. Guidance and Assistance. The Master Scheduler should seek advice and
assistance from appropriate supervisors regarding improved work methods,
better job sequencing, the possibility of overtime use, additional personnel
or shifts, special equipment, or more advantageous material.
c. Trial and Error. Any decrease in time of any arrow (step) on the
critical path will decrease the total duration time for the job. As with the
arrow diagram, a trial and error method is required before the final plan is
developed. The initial critical path for shop forces consists of steps 4, 8,
9, 10, and 14. One way to shorten the time would be to increase the number of
craftsmen on each step. Doubling the crew on steps 4 and 14 and tripling the
crew on steps 8, 9, and 10 would reduce the time from 285.3 hours to 105.9
hours. This increase in crew size appears reasonable because of work space
A-8
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