M2.2 — Logic, Critical Path & Float

M2.2 — Logic, Critical Path & Float

1. Practical outcome

The learner should be able to construct and analyse a logically linked schedule, choose relationships, recognise missing or inappropriate logic, distinguish logic from constraints, identify critical and near-critical paths, understand total and free float, recognise negative or unusually high float, understand how progress changes the path, and use the analysis to support decisions. The goal is to understand the schedule as a model, not simply as software output.

2. Core concepts

·       Finish-to-Start (FS): B cannot start until A finishes. Example: Design → Manufacture.

·       Start-to-Start (SS): B cannot start until A starts. Example: excavation/removal work may legitimately overlap.

·       Finish-to-Finish (FF): B cannot finish until A finishes.

·       Start-to-Finish (SF): the successor cannot finish before the predecessor starts, adjusted for any lag. This unusual relationship needs a clear operational justification.

·       Leads and lags modify timing between linked activities. Ask what the lag represents; if it represents measurable work or an event, model that explicitly where practicable. Avoid negative lags (leads) that assume future accomplishment. Where genuine overlap is needed, use measurable hand-offs and appropriate relationships, documenting any justified exception under the adopted scheduling rules.

·       Constraints are different from logic. The practitioner should distinguish logic-driven dates from constraint-driven dates.

3. Step-by-step method

1. Understand the work, including physical, technical, contractual, external and approval dependencies.

2. Establish predecessors and successors. Most activities should have both, except legitimate project starts and finishes. Unexplained missing links are open ends.

3. Select the appropriate relationship. Model how the work is actually expected to be delivered rather than using relationships merely to force overlap or dates.

4. Use leads, lags and constraints only where there is a defensible reason.

5. Calculate the network. Understand the idea of a forward pass and backward pass even when software performs the calculation.

6. Review open ends, hard constraints, long lags and unusual relationships.

7. Identify the current critical path and important near-critical paths.

8. Review total and free float and investigate unusually high or negative values.

9. Trace the path operationally and confirm that it makes practical sense.

10. Reassess the path structure after each progress update because the controlling path can change.

4. Worked example

Consider a simplified sequence using one working-day calendar, finish-to-start relationships with zero lag, no imposed date constraints and sufficient resources. Day 0 is the common start:

·       A — Complete design: 5 days

·       B — Procure equipment: 10 days, dependent on A

·       C — Prepare site: 6 days, dependent on A

·       D — Install equipment: 5 days, dependent on both B and C

·       E — Test and commission: 3 days, dependent on D

Path A–B–D–E = 23 working days. Path A–C–D–E = 19 working days. A–B–D–E is the critical path. C has four working days of total float and four of free float under these assumptions.

If C finishes five working days later while B remains on plan, A–C–D–E becomes the 24-day controlling path and completion slips by one working day. Four days consume the available float; the fifth affects completion. Recalculate the network to verify the result.

Float

·       Total float is the movement available before a relevant network completion or constrained date is affected, subject to the way the schedule is configured. It is not simply “days to waste”.

·       Free float is the delay available without delaying any immediate successor’s early dates, as determined by the relationship type and calendar. In a simple finish-to-start network this is the gap to the successor’s early start.

·       Negative float indicates that the schedule cannot currently achieve an imposed requirement or constrained date. Investigate the requirement, controlling path and driver; do not simply add constraints to hide the result.

Critical and near-critical paths

Do not simply accept red bars or software labels. Trace the path, check continuity, check whether it makes operational sense, examine constraints, calendars and lags, and confirm that the expected delivery drivers are present. Near-critical paths matter because relatively small changes can make them controlling.

The lowest-total-float path and the longest remaining path need not coincide when constraints, multiple calendars, out-of-sequence progress or resource levelling affect the calculation. Explain which path drives the milestone being assessed and retain the calculation settings with the schedule basis.

Progress and path change

The critical path can change as the project progresses. If procurement remains on plan but site preparation suffers a five-day delay, the previously non-critical site path may become the controlling path. Critical-path analysis should therefore be repeated at each update.

A better question than “What is the critical path?” is: “What is currently controlling important outcomes, why, and what might become controlling next?”

5. Interpretation and professional judgement

The schedule calculation is not the end of the analysis. The practitioner must decide whether the calculated path represents the real delivery mechanism. A path driven by artificial constraints, unrealistic calendars or missing logic can be mathematically valid and operationally misleading.

Negative float should trigger investigation of the requirement and the controlling sequence. High float should be challenged because it may indicate genuine flexibility, but it may also indicate missing logic or disconnected scope. Automated health checks identify symptoms; professional judgement determines whether the model is appropriate.

6. Practice diagnostic

Use this table to challenge the quality of the approach or deliverable. The rows consolidate the most important recurring red flags and checks; they are prompts for professional judgement rather than a substitute for the method above.

7. Try-it-yourself practical exercise

Build a 15–20 activity network for part of the Reference Project. Assign relationships and identify the critical path and float. Delay a non-critical activity until its float is consumed and recalculate. Delay another activity so that a different path becomes critical. Explain what changed and why. Then introduce one inappropriate hard constraint, observe the effect, remove it and restore credible logic.

The learner should understand WHY the schedule moved, not merely which button was pressed.

8. Bridge to capability

The How-to teaches how schedule logic, critical paths and float behave. At APPLIED and above, capability assessment should test whether the practitioner can apply the method to a realistic network, identify weak logic, explain the consequences and recommend corrective action. A checked network and before-and-after calculations should demonstrate how assumptions and progress change the controlling path. Supporting work products are illustrative and should be assessed with the practitioner’s reasoning. Sample learning questions illustrate the judgement at each recognition level; they are discussion prompts rather than validated assessment items.

Learning question: Do you understand how logic, critical paths and float affect schedule outcomes?

Capability question for APPLIED and above: Can you demonstrate a defensible network analysis, explain changing path behaviour and recommend a justified correction?