Analyzing Schedule Feasibility and Risk

Lecture Notes: Applying Leads, Lags, and Constraints

Introduction

In real-world projects, activities rarely flow in a perfect one-after-the-other pattern. Some tasks can start early, others need a buffer, and a few must wait for fixed dates or conditions.
That’s where leads, lags, and constraints come in — fine-tuning tools that make schedules realistic and flexible.

Used well, they make the difference between a theoretical plan and one that matches how work actually unfolds.

 Leads and Lags: Adjusting Activity Timing

Lead

A lead allows a successor activity to start before its predecessor finishes.

• It accelerates the schedule.
• Common in overlapping or fast-tracked work.

Example:
If “Testing” can begin 2 days before “Coding” finishes, there’s a 2-day lead.

Effect: Shortens the total project duration.

Lag

A lag is a delay between the completion (or start) of one activity and the beginning of another.

• It adds waiting time.
• Often used when tasks require curing, drying, or approval periods.

Example:
If “Concrete Curing” starts 3 days after “Pour Concrete” finishes, there’s a 3-day lag.

Effect: Extends the project timeline or builds intentional buffers.

Applying Leads and Lags in Practice

Note:

• Avoid excessive use of leads and lags — they can make dependencies harder to trace.
• Document every lead or lag with a clear rationale.

Constraints: Controlling the Timing of Activities

A constraint restricts when an activity can start or finish.
They are often used to reflect external conditions, contractual dates, or resource limits.

Common Constraint Types (in scheduling software like MS Project):

When to Use Constraints

• When an external dependency exists (e.g., regulatory approval, client event).
• To align schedules with contracts or fiscal calendars.
• For milestones with non-negotiable dates.
• To anchor key activities in the timeline.

But:
Overusing “hard” constraints (MSO, MFO) limits flexibility and may cause negative float — making schedule control harder.

Balancing Flexibility and Control

Example Scenario

Context:
In a construction project:

• “Install wiring” must start 2 days before “Wall completion” (lead = 2 days).
• “Paint walls” must start 3 days after “Plastering” (lag = 3 days).
• “Final inspection” must finish by June 30 (FNLT constraint).

These settings make the plan more realistic — reflecting overlap, waiting time, and fixed deadlines.

Common Mistakes

• Using leads to “hide” delays or compress schedules unrealistically.
• Applying too many constraints, freezing flexibility.
• Forgetting to update or remove outdated lags.
• Misinterpreting leads/lags as resource availability buffers (they’re not).

Conclusion

Leads, lags, and constraints are the project manager’s fine-tuning tools — shaping the rhythm of a schedule.
Used wisely, they balance accuracy with flexibility; used poorly, they create confusion and false urgency.

A schedule isn’t just about what follows what — it’s about when, and under what conditions, each piece can truly move.