Constraints are used to artificially modify or limit the dates of a milestone or activity. Ideally, a well-constructed schedule should not have any restrictions. However, in practice, constraints find their way into work baselines and updates for various reasons. The only instances where constraints may be justified are when the scope of work involves a third party, such as the delivery of materials falling outside the project’s scope. Constraints can also be useful for controlling the margin (float) of specific activities or work structures.
Hard Constraints
- Mandatory Start/Finish – This type of restriction forces the start or end of an activity at at a specified date and time, violating the relationships between activities and even working times. It can even initiate an activity on a holiday or before its predecessor is finished.
Soft Constraints
- As Late as Possible: By default, activities are scheduled “as soon as possible.” This restriction delays the start of an activity, utilizing any available free float. If the free float is zero, the dates remain unchanged (For Critical activity, early start = late start).
- Start/Finish On: This allows you to designate a specific start or finish date for an activity. However, it’s essential to note that this restriction is subordinate to relationships and calendars.
- Start/Finish On or After: This constraint enables you to specify a date from which an activity can commence or conclude. As with the previous constraint, relationships and calendars take precedence over this restriction.
- Start/Finish On or Before: This allows you to identify the latest possible date an activity can start or finish. Similar to the others, relationships and calendars hold sway over this constraint.
The key distinction between “Start/Finish On” and “Start/Finish On or Before/After” lies in their impact on activity criticality. “Start/Finish On” always makes an activity critical, while the other types of restrictions, dependent on dates and relationships, may or may not affect an activity’s critical status.