Time Impact Analysis and Windows Analysis: choosing an appropriate method

Time Impact Analysis and Windows Analysis are recognised approaches, but they are designed for different analytical circumstances. Choosing between them, or combining them, is a matter of matching the analysis to the question being answered, the timing of the assessment, the available programmes and records, the contractual mechanism, the actual progress information and the intended use of the findings.

This article explains what each approach does, what each depends on, and a practical set of factors for selecting an appropriate method.

This article provides general technical information on programme and delay analysis. Contractual and legal consequences depend on the applicable contract, amendments, governing law and facts, and should be reviewed with the appropriate legal advisers.

Two approaches, two questions

Time Impact Analysis

TIA models a delay event as a fragnet, a small network of activities representing the event, inserted into a programme, and measures the resulting movement of completion. It commonly assesses the forecast programme effect of an event using the programme position relevant to the assessment, which is why the programme selected for the analysis matters: its data date, logic, recorded progress and contemporaneous information carry as much weight as the fragnet itself.

Prospective contractual mechanisms may require forecast assessment. Under NEC, compensation events are generally assessed on a forecast basis under the relevant contractual mechanism, and the applicable accepted programme may depend on the event and the notification process; the contract wording governs, and its interpretation is a matter for the legal advisers.

Used retrospectively, long after the events, TIA requires care: the analysis should explain why a forecast-based assessment remains appropriate and how its results relate to what actually happened. The SCL Protocol discusses prospective and retrospective approaches and the considerations relevant to each; its guidance is best read directly rather than reduced to a single preference.

Windows Analysis

Windows or time-slice analysis divides the project into meaningful periods, often the update cycle, and examines the changing completion forecast, the critical and near-critical paths, and the progress and events within each period, connecting material findings to the project records.

Its requirements are real: a suitable baseline and a run of usable updates, reliable progress information, consistent data dates and status, and attention to logic and constraints. Where updates are missing and windows are reconstructed from the records, the reconstruction should be transparent and the assumptions identified.

Choosing a method: five factors

1. The question to be answered. A forecast assessment of an event and an explanation of a completed project's delay are different questions.
2. Timing of the assessment. Contemporaneous assessment during delivery and retrospective analysis after the events call for different approaches.
3. Programme and record quality. The available baselines, updates and progress records constrain what any method can support.
4. The contractual mechanism. Where the contract prescribes how delay is assessed, the analysis should serve that mechanism.
5. Intended audience and use. The analysis should be proportionate and understandable for its purpose, whether that is a negotiation, an adjudication or another forum. A more elaborate method is not automatically better; an analysis that cannot be followed by its audience serves no one.

Guidance and professional judgement

The SCL Delay and Disruption Protocol and AACE International Recommended Practice 29R-03 are reference materials, not rigid method-selection rules. AACE 29R-03 classifies and describes forensic schedule analysis techniques rather than prescribing a single standard, and neither document removes the need for professional judgement. A considered departure from guidance is not automatically an error where it is explained: the report should say what was done, why, and what the limitations are. How we select and explain methods is set out on our Approach page.

Using more than one approach

A matter may legitimately use more than one analytical approach: for example, a forecast-based assessment for events assessed under the contract during delivery, alongside a retrospective analysis of the project as a whole. Where multiple analyses are used they should answer clearly identified questions, use compatible factual assumptions, avoid double counting, reconcile material differences and explain why each method has been used. The report should explain why each approach is suitable, what information it relies upon and what limitations affect the result.

Method selection is part of the analysis. It should be explained by reference to the question, the records and the contractual context.

References and further reading

• Society of Construction Law, Delay and Disruption Protocol, 2nd Edition (2017): scl.org.uk
• AACE International, Recommended Practice 29R-03, Forensic Schedule Analysis: aacei.org
• Official NEC contract guidance on compensation event assessment: neccontract.com

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