This post provides methods of allocating time contingency, that may be useful when constructing or reviewing project schedules. Note – guidance in determining time contingency values is beyond the scope of this article.
Contingency vs Float
For the purposes of this article two definitions are to be noted:
- Contingency and float are not the same, float is a result of CPM calculations, whereas contingency is explicit duration(s) assigned to the project schedule
- Contingency duration(s) are additional duration beyond what would be considered as “normal” or the “expected” duration.
Contingency has the effect of reducing schedule float.
Method One – Additional Duration to Activities
This method extends activity duration such that Total Duration = “Normal” Duration + “Contingency” Duration. This method can be applied to any selected activity within the schedule as shown in the diagram below:
The value of contingency added to activities can be easily determined (eg. as a fixed value or % of normal duration).
Method Two – Additional Calendar Non-Work Periods
This methods includes contingency by adding extra non-work periods into activity calendar. This has the effect of extending the period over which activities occur, without any additional activity duration as shown below:
This method also allows contingency to be varied according to varying calendars, and can be documented by identifying the contingency non-work periods in a calendar register.
Both method one and two have the following issues:
- No explicit indication of contingency at activity or project level
- Applying this method to non-critical activities will not provide overall project contingency
- The schedule dates will not reflect the expected start and finish dates.
- As the project progresses, transfer of contingency is complicated, and requires discipline
Method Three – Discrete Contingency Activities
This method inserts additional activities into the schedule, explicitly for the purpose of indicating time contingency. Typically this activity is inserted as the last activity in a sequence of works leading to a completion milestone, but may exist elsewhere. This may occur to a single completion milestone, or to several completion milestones if appropriate for the project. The purpose of this method is to clearly demonstrate what value of contingency is included into a schedule, as per the figure below:
Method three provides a number of benefits:
- Value of contingency duration can be readily determined, documented and applied
- The schedule will clearly presents the contingency value
- Contingency can be inserted as required into the schedule, ie specifically into critical path if required.
- Schedule will present “normal” dates that can be expected until the contingency activity occurs
- Transfer of the contingency by modifying the contingency duration is performed easily.
Issues with this method can include:
- Amount included for contingency will be explicit and therefore open to political interpretation
- Will affect float calculations
- Can be mistaken to represent float
There exist other methods to introduce contingency into a schedule, by manipulating schedule logic, however these will result in major diversions of the schedule away from what is “normal” or “expected” and should be used with caution. Some examples are briefly provided below:
Method Four – Relationships
This method adds contingency by sequencing activities such that they may not reflect the true relationship between them. Typically this is achieved by using Finish-to-Start relationships between activities, while the work may otherwise occur in some parallel sequence, as shown in the following figure:
Method Five – Relationship Lags
This method adds contingency by introducing lags onto relationships that may not otherwise require them. This results in activities maintaining their “normal” durations, however the start and finish dates they occur at will be impacted by the contingency included as relationship lags, refer to the figure below: