Lags, leads and date constraints in a project schedule
A lag is a wait between two activities that uses no effort or resources, and a lead is a negative lag. A date constraint overrides network logic to fix when an activity may start or finish. GAO says all three should be used sparingly and justified, and the baseline schedule should not contain hard constraints.
Lags and leads
A lag delays a successor for a passage of time with no effort or resources, such as waiting for concrete to cure before stripping forms. A lead accelerates a successor so it overlaps its predecessor.
GAO warns that lags are static: they must be changed by hand and cannot be statused. They are often misused to force a start date or as risk buffers, and can distort float and the critical path. A lag that stands for a waiting period, such as document review, should be replaced with an activity that can be monitored, statused and mitigated.
GAO's example: a 2-day paperwork lag after an HVAC inspection uses up the inspection's 2 days of float, so the inspection shows as critical. Effort aimed at the inspection would be misdirected, because the lag causes the criticality.
Leads assume foresight about future events. GAO prefers to split the predecessor into smaller activities and use F–S links.
Types of date constraint
Soft constraints are start no earlier than (SNET) and finish no earlier than (FNET). They stop an activity moving earlier but let it slip later if predecessors are late.
Hard constraints are start no later than (SNLT), finish no later than (FNLT), must start on (MSON) and must finish on (MFON). The 'must' constraints set early and late dates equal, which removes all float and makes the activity critical at once.
When constraints are justified
An SNET constraint is valid when an activity cannot start before a fixed date and has no other logical dependency, for example when an outside party will not release a facility before a set date.
A customer-mandated date, including a contractual obligation, is not by itself a reason to constrain an activity. Ideally only the program start date is entered and every other date comes from logic and durations. Working schedules may use hard constraints to test float, but the baseline and official status updates should not contain them. Every lag and constraint should be justified in the schedule documentation.
Where the exam catches people
Believing a contract date should be entered as a must-finish-on constraint in the baseline: GAO says the baseline should not contain hard constraints.
Believing an SNET constraint stops an activity slipping: it only stops it starting early.
Believing a waiting period is best shown as a lag: GAO prefers an activity that can be monitored and statused.
Believing an activity shown as critical is always the real driver: a successor lag can make it look critical.
Believing a lead is a reliable way to overlap work: GAO prefers splitting activities and using F–S links.