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.2 Key Terms and Definitions
Work Breakdown Structure
A Work Breakdown Structure (WBS) is a 'family tree' of the project tasks and
deliverables required to deliver the final product. It is usually broken down into phases,
activities, and tasks as illustrated below:
Elapsed Time vs. Duration vs. Effort
There is often considerable confusion when discussing the differences between elapsed
time, duration, and effort. The following defines these terms and shows the equivalent
relationship to the MS Project tool.
Elapsed Time
This is the elapsed calendar time (days, weeks, months, etc.), including holidays
and other non-working days, required to complete a given activity. In MS Project, this is
the number of days between the scheduled start and finish dates of a task.
The Construction Industry often refers to elapsed time as "Calendar Days".
Duration
This is the amount of time (hours, days, weeks, months, etc.), not including
holidays and other non-working days, necessary to complete the work required for a given
activity. MS Project utilizes a 'Duration' field that is consistent with this definition.
The Construction Industry often refers to duration as "Work Days".
Effort (Work)
This is the number of work units required to complete a given task. Work units are
usually expressed as hours, days, or weeks. "Effort" in MS Project is called
"Work".
Effort and Duration are intricately tied together. For example:
| A 5-day task (effort), where the resource is working at 100% availability on the task,
will have a duration of 5 days. |
| A 5-day task (effort), where the resource is working at 50% availability on the task,
will have a duration of 10 days. |
| This concept is covered in further detail in Step 5 of this technique. |
NOTE: Resource availability in MS Project is defined by using the "Resource
Max. Units'" field. If the Resource Max. Unit = 1, the resource is allocated at 100%.
If the Resource Max. Unit = 0.5, the resource is allocated at 50%.
Gantt vs. PERT
The Gantt Chart and PERT Chart are two distinct tools for displaying schedule
information. Each serves a specific purpose when building the project schedule. Both of
these charts are readily available in MS Project.
Gantt Chart
This is a graphic display of schedule-related information. In the typical Gantt
Chart, tasks are listed down the left side of the chart, dates are shown across the top,
and activity elapsed times are shown as date-placed horizontal bars. The following are
examples of a Gantt Chart:
Click the "thumbnail" image below to view a full size image of the Gantt Chart.
PERT Chart
The PERT Chart uses a network diagramming technique in
which tasks are represented by boxes. Tasks are linked by logical relationships or
dependencies to show the sequence in which the tasks are to be performed. The PERT Chart
in MS Project also automatically calculates the Critical Path. The Critical Path is the
sequence of tasks (or path) that will take the longest to complete. The Critical Path may
change from time to time as tasks are completed ahead of or behind schedule. The following
is an example of a PERT chart:
Click the "thumbnail" image below to view a full
size image of the PERT Chart.
Dependency
A dependency is a logical relationship between two project tasks, or between a project
activity and a milestone. The four possible types of logical relationships are as follows:
| Finish to start -The 'from' activity must finish before the
'to' activity can start. |
| Finish to finish - The 'from' activity must finish before
the 'to' activity can finish. |
| Start to start - The 'from' activity must start before the
'to' activity can start. |
| Start to finish - The 'from' activity must start before the
'to' activity can finish. |
Lag Time
This is a modification in the dependency where there is a delay in the start of the
successor task. For example, in a finish-to-start dependency, with a 10-day lag, the
successor activity cannot start until 10 days after the predecessor has finished.
Lead Time
This is a modification in the dependency where there is acceleration in the start of
the successor task. For example, in a finish-to-start dependency, with a 10-day lead, the
successor activity can start 10 days before the predecessor has finished.
Slack Time
This is the amount of time that an activity may be delayed from its early start without
delaying the project finish date. Slack can change as the project progresses and changes
are made to the project plan.
Milestone
A milestone is a significant event in the project, usually completion of a major
deliverable. Milestones may also be used to produce summary level schedules (called
'Master Schedules') that depict major tasks and key milestones. In MS Project, milestones
are typically represented as tasks with zero duration.
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