The forward pass

During the forward pass, earliest dates are recorded as they are calculated. For events, they are recorded on the network diagram and for activities they are recorded on the activity table.

The forward pass is carried out to calculate the earliest date on which each event may be achieved and the earliest date on which each activity may be started and completed. The earliest date for an event is the earliest date by which all activities upon which it depends can be completed.

By convention, dates indicate the end of the a period and the project is therefore shown as starting in week zero (or the beginning of week l).

The forward pass and the calculation of earliest start dates is calculated according to the following reasoning.

• Activities A, B and F may start immediately, so the earliest date for event l is zero and the earliest start date for these three activities is also zero.

• Activity A will take 6 weeks, so the earliest it can finish is week 6 (recorded in the activity table). Therefore the earliest we can achieve event 2 is week 6.

• Activity B will take 4 weeks, so the earliest it can finish and the earliest we can achieve event 3 is week 4.

• Activity F will take 10 weeks, so the earliest it can finish is week 10 - we cannot, however, tell whether or not this is also the earliest date that we can achieve event 5 since we have not, as yet, calculated when activity E will finish.

• Activity E can start as early as week 4 (the earliest date for event 3) and, since it is forecasted to take 3 weeks, will be completed, at the earliest, at the end of week 7.

• Event 5 may be achieved when both E and F have been completed, that is, week 10 (the later of 7 and 10).

• Similarly we can reason that event 4 will have an earliest date of week 9. This is the later of the earliest finish for activity D (week 8) and the earliest finish for activity C (week 9).

6.13 The backward pass

The earliest date for the completion of the project, event 6, is therefore the end of week 13 - the later of 11 (the earliest finish for H) and 13 (the earliest finish for G).

The results of the forward pass are shown in Figure 6.18 and Table 6.2.

Figure 6.18 A CPM network after the forward pass.

Table 6.2

The activity table after the forward pass

Table 6.2

Activity

Duration

Earliest

Latest Earliest Latest

Total

(weeks)

start date

start date finish fate finish date

float

A

6

0

6

B

4

0

4

C

3

6

9

D

4

4

8

E

3

4

7

F

10

0

10

G

3

10

13

H

2

9

11

The forward pass rule: the earliest date for an event is the earliest finish date for all the activities terminating at that event. Where more than one activity terminates at a common event we take the latest of the earliest finish dates for those activities.

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