21-06-2012, 01:33 PM
Basic CPM Calculations
Basic CPM Calculations.pdf (Size: 278 KB / Downloads: 32)
Overview
Time Analysis calculations in a Precedence Diagramming Method (PDM) Critical Path
Method (CPM) network can be done in a number of different ways. The purpose of this
White Paper is to offer a standardised set of calculations and note areas where different
software tools may produce different outcomes.
Basic Definitions & Key
This White Paper uses a standard Activity Box laid out as per the ‘Key’ below. The layout of
Activity nodes in commercial scheduling software is highly configurable; care needs to be
taken to understand the format being used.
PDM Activities, Links and logic
Precedence diagrams use boxes to represent the basic network elements - the activity (or task)
and lines to show the inter dependency (or links), between the activities to define the flow of
work.
In precedence networks, activities:
- are identified by a activity identifier; for example, A1, A2, A3.
- have durations defining the period of time required to perform the work2, and
- may have other descriptive data attached to them.
Links are usually identified by their preceding activity identifier and their succeeding activity
identifier. Most links simply represent the flow of work from the completion of one activity to
the start of the next. However, the effect of a link may be modified by modifying the type of
link used and/or the addition of a ‘Lead’ or a ‘Lag’.
Progress Calculations
The same basic calculations described above are used to calculation the situation after a status
update10. Typically actual start dates are recorded for all activities actually started. Finish
dates for all activities actually complete and the amount of time required to complete all inprogress
activities. The time to complete is assessed on the same basis as the duration, the net
amount of working time. All incomplete work is then re-scheduled from the status date or
time.
Out-of-Sequence Progress
When statusing and updating a schedule to record progress, particular care is needed to ensure
the logic between overlapping activities is still appropriate for the actual progress of the work.
Many tools offer the option to either maintain logic (preferred) or override logic. Before
analysing the schedule, the best general setting should be selected and then specific
adjustments are made to ensure the network accurately models the intended pattern of work.
The example below shows the same set of activities with ‘logic override’ selected on the left,
and ‘maintain logic’ selected on the right.