Project Services Company Inc, P.O. Box 24628, Tempe, AZ  85285-4628 U.S.A.Tel: (408) 820-2235  Fax (408) 820-4612
Some Basics of CPM Scheduling
CPM scheduling is based on a network of project activities that is established to model or mimic the construction process that is required to build a project. Typical activities include foundations, masonry, roof tile, install carpet, etc. Obviously some of these activities depend on others being done first. The network formalizes the sequence in which the activities should be performed.  However, in order to result in a schedule, either estimated or calculated durations are assigned to each activity.
Every activity in the network, except the first and last, are linked to specific other activities that effect the start and effect the finish of that activity.  These links are called relationships and those effecting the start of the activity are termed Predecessors (because they come first) and those effecting the finish of the activity are termed Successors (because they follow the activity).
The main types of relationships are:
          
Finish to Start (FS).  The first activity must be finished  before the second can start. This is the simplest and most common type of relationship. Example: Wall -> Roof
Start to Start (SS). The second activity can start after the first has started, but it is not necessary that the first activity be finished before the second starts. Example: Hang drywall -> texture drywall
Finish to Finish (FF). The second activity can not finish before the first has finished. Example: Suspended ceiling grid -> Acoustic ceiling tile
Lags are planned time intervals between activities and are used with all types of relationships. The normal lag is 0, however SS and FF relationships frequently have lags greater than 0. Example: Hang drywall -> SS 2 -> texture drywall.
Lags can be positive or negative
Overlapping activities can be handled either by using a double relationship between activities ( an SS and an FF relationship with lags as appropriate) or by using a FS relationship with a negative lag.  Microsoft Project does not support double relationships between the same two activities.
The software performs the critical path calculation by adding up the time requirements along each leg of the network, starting from the first activity. This calculation is called the forward pass. The longest calculated path to completion is the Critical Path. The software calculates the start and finish dates for all the activities as part of the calculation.
After performing this calculation, the software then adds up the time requirements again, starting from the last activity going to the first. This is termed the backward pass.  The calculated dates along the critical path obviously will be the same as for the forward pass, but every other activity will have a second set of dates which reflect the latest dates by which these activities have to be started and finished. The difference between the two sets of dates is called the float (or slack), which is zero for the critcal path and a positive number along all other paths.
Most software programs permit the imposition of various constraints on selected activities. This includes a fixed completion date and other immovable dates during the project.
A schedule that is developed and when required, approved, at the start of a project is a Baseline schedule. This is generally an important project document.
As the construction work proceeds the baseline schedule is updated by adding the actual dates when work items were started and completed, and the schedule is recalculated.  Each updated version should be subsequently updated again until the project is completed.  If there are constraints on some activities including on completion, negative float may develop during updating. This basically is the measure of the delay in complying with the constraints.
A baseline or updated schedule becomes a revised schedule when relationships or planned durations are changed or activities are added or deleted. Revisions may be necessary when negative float develops, or when the completion of a schedule without constraints is slipping.
Delays and delay impacts are of two types. Project delays occur when the project completion is impacted and activity delays when only one or a series of activities are delayed but there is no impact on project completion. Project delays are the most serious type and the subject of most successful delay claims. Establishing the extent of the delays can be a complex process of comparing the project status before and after the impact occurred. There can be a series separate project delays in the course of executing a large project.
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Project Services Company Inc, P.O. Box 24628, Tempe, AZ  85285-4628 U.S.A.Tel: (408) 820-2235  Fax (408) 820-4612
Some software programs offer a feature called progress override that overrides the original logic when the updating indicates the activities were actually executed in a sequence different from that in the schedule. To not override the original logic is termed retained logic.