Other Scheduling Methods - Construction Project Scheduling and Control - page 306

and

T S =σ E Z + T E = 3 . 531 . 645 + 54 . 83 = 60 . 64days

Let us discuss the results: when we look at the most likely durations (as
single-value, or deterministic, durations), path ABDHI is the longest, at 55
days. When we look at the expected durations, path ABDG is the longest, at
56.33 (say, 57) days. When we consider the standard deviation and the 95%
required confidence level, the same path, ABDG, is longest (by a larger margin),
at 64.41 (say, 65) days. Note also that in this case, path ABEHI is second in
importance (at 60.64 days), after path ABDG and before path ABDHI. In other
words, the path considered longest by the deterministic CPM calculations was
superseded by two other paths that were considered by means of PERT to be
more critical.
Note : The preceding calculations were done using an Excel spreadsheet.
The answers were rounded for simplicity.
Example 11.2 illustrates the importance of considering near-critical paths,
especially those with high uncertainty (a large standard deviation). Simulations
may be used to determine which path is the most critical.

Using PERT to Calculate the Date of an Event with a Certain
Level of Confidence
The main focus in PERT is the event, not the activity. 2 You can use the same probabilis-
tic approach with any event (e.g., delivery of a custom item, the building is watertight,
substantial completion). In this case, you consider the path from the start of the project
(or data date, if project has already started) until, and including, the event under
consideration.

Example 11.3

Calculate the date of completion of activity F in example 11.2 with 95%
confidence.

Solution

From the calculation table in example 11.2, we take the path ACF.

2 The terms event and activity were introduced in Chapter 3. An activity has a start date, a duration, and a
finish date. An event has just one date (start or finish) with no duration. An event can be the start or finish of
an activity.