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Part Three
Planning and control
Figure 16.16 A network diagram for the project ‘decorate apartment’ with earliest and latest
event times
also has event number 5 as its head event so we must also calculate the EET of activity e’s tail
event. This is determined by activity d. If activity d starts at day 1 (the earliest it can) it will
finish at day 2. So the EET of event number 4 is day 2. If activity e is started immediately,
it will then finish at day 4. Event number 5 cannot occur, however, until both e and c have
finished, which will not be until day 6 (see rule 1 above). Activity f can then start and will
finish at day 7.
The LETs can be calculated by using the reverse logic. If event number 6 must occur no
later than day 7, the LET for event number 5 is day 6. Any later than this and the whole project
will be delayed. Working back, if activity c must finish by day 6 it cannot start later than
day 3, and if activity b must finish by day 3 it must start by day 1. Similarly, if activity e is to
finish by day 6 it must start no later than day 4, and if activity d is to finish by day 4 it must
start no later than day 3. Now we have two activities with event number 2 as their tail event,
one of which needs to start by day 1 at the latest, the other by day 3 at the latest. The LET for
event number 2, therefore, must be the smaller of the two. If it were delayed past this point,
activity b, and therefore the whole project, would be delayed.
Worked example
The chief surveyor of a firm that moves earth in preparation for the construction of
roads has identified the activities and their durations for each stage of an operation to
prepare a difficult stretch of motorway (see Table 16.3). The surveyor needs to know how
long the project will take and which are the critical activities.
Table 16.3 Road construction activities
Activity Duration Preceding
(days)
activities
A 5 –
B 10 –
C 1 –
D 8 B
E 10 B
F 9 B
G 3 A, D
H 7 A, D
I 4 F
J 3 F
K 5 C, J
L 8 H, E, I, K
M 4 C, J