How does a project manager calculate start and finish times?
How does a project manager calculate earliest completion time (ECT) and latest completion time (LCT) for the tasks in a project?
ESTIMATING TASK DURATION Task duration can be hours, days, or weeks — depending on the project.
Because the following example uses days, the units of measurement are called person-days.
A person-day represents the work that one person can
complete in one day. This approach, however, can present some problems. For example,
if it will take one person 20 days to perform a particular task, it might not be true that
two people could complete the same task in 10 days or that 10 people could perform
the task in two days. Some tasks can be divided evenly so it is possible to use different
combinations of time and people, up to a point. For instance, if it takes two persondays
to install the cables for a new local area network, one person might do the task in
two days, two people in one day, or four people in half a day. In most systems analysis
tasks, however, time and people are not interchangeable. If one analyst needs two hours
to interview a user, two analysts also will need two hours to do the same interview.
Project managers often use a weighted formula for estimating the duration of each
task. The project manager first makes three time estimates for each task: an optimistic,
or best-case estimate (B), a probable-case estimate (P), and a pessimistic, or worst-case
estimate (W). The manager then assigns a weight, which is an importance value, to each
estimate. The weight can vary, but a common approach is to use a ratio of B = 1, P = 4,
and W = 1. The expected task duration is calculated as follows:
(B+4P+W)
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For example, a project manager might estimate that a file-conversion task could be
completed in as few as 20 days or could take as many as 34 days, but most likely will
require 24 days. Using the formula, the expected task duration is 25 days, calculated as
follows:
(20+(4*24)+34) =
6 25
Factors Affecting Duration
When developing duration estimates, project managers consider four factors:
• Project size
• Human resources
• Experience with similar projects
• Constraints