Windows analysis

Windows analysis looks at a project one period at a time. For each window, it compares the schedule updates at the start and end, finds how much the critical path moved, and identifies which events drove the movement. Because it uses the updates as they were, it captures the delays of every party, including the one making the claim. That is why it is often the method of choice for reviewing a contested claim.

Learning outcomes

• Explain how windows analysis works and why it uses contemporaneous updates.
• Set window boundaries that make sense for the project.
• Assign the delay in each window to owner, contractor, neutral or concurrent causes.
• Reconcile the windows to the total project delay.

The lesson

How it works

Divide the project into windows, usually between accepted updates or at major milestones. In each window, compare the forecast completion at the start and end. The change is the delay, or recovery, in that window. Then trace the critical path through the window to find which events caused the movement. In AACE RP 29R-03 terms, using the updates as they were is observational, dynamic logic (MIP 3.3 or 3.4); recreated updates fall under MIP 3.5 [1].

Setting windows

Windows should match the records: monthly windows if the updates are monthly and reliable, longer windows around milestones if not. Boundaries should not split a single event awkwardly. Short windows show more detail but take more effort.

Assigning delay

For each window, list the events on or near the critical path and decide which drove the movement. Where an owner event and a contractor event drove it over the same days, record the days as concurrent. Where the path recovered, record that too: recovery is part of the picture. The windows must add up to the total delay. If they do not, something is missing.

Strengths and limits

Windows analysis captures every party's delays and follows the critical path as it actually shifted. Its quality depends on the updates: unreliable updates must be corrected or recreated first, and the corrections must be explained. Courts look for this kind of critical path reasoning; in Schindler v. Walsh, the Ontario Superior Court relied on the critical path as the longest chain of logically connected activities [2].

Northgate worked example

Northgate Distribution Centre (fictional) is a 12,000 m² tilt-up warehouse in Vaughan, Ontario, built by Halvorsen Builders Ltd. (fictional) under CCDC 2-2020, with Ridgeline Architects Inc. (fictional) as Consultant. See the Northgate fact sheet. This course follows one delay claim from first notice to settlement. Ready-for-Takeover (M9) was due Friday, January 14, 2028 and was reached on Tuesday, March 14, 2028: 60 calendar days late. All events, dates and amounts are illustrative.

Northgate had eleven accepted monthly updates in native P6 format, so the owner's representative uses four windows built from them (Figure 2):

View the data

Item	Value
W1 Mar 1 to Jun 30, 2027	8 days
W2 Jul 1 to Sep 30, 2027	15 days
W3 Oct 1 to Nov 30, 2027	14 days
W4 Dec 1, 2027 to Mar 14, 2028	23 days

The windows reconcile to the 60 days of actual delay. Totals by category: owner 18, neutral 14, concurrent 10, contractor 18. The owner's representative shares the full analysis, with native files, with Halvorsen's scheduler. That transparency matters: it moves the negotiation from "whose method" to "which days", and in June 2028 the schedulers agree the windows with only minor changes.

Your move

If you are the owner

• Keep updates clean and accepted. They are the raw material for the windows.
• Share the analysis. A transparent analysis is harder to attack and easier to settle on.
• Record recovery as well as delay. It is part of a fair picture.
• Reconcile to the total. Every day must be in a window.

If you are the contractor

• Run windows on your own claim before submitting. If the result differs from your impacted as-planned, expect the owner's version.
• Explain corrections to updates. Unexplained changes destroy credibility.
• Show your delays and your recovery. Recovery you funded may support an acceleration claim.
• Agree window boundaries early. It narrows the dispute.

Common mistakes

1. Windows that don't add up. Missing days undermine the whole analysis.
2. Silently corrected updates. Every change must be documented.
3. Windows that split events. Boundaries should follow the records and the events.
4. Ignoring concurrency inside a window. Two drivers over the same days are concurrent.
5. Arguing method instead of days. Agree the method, then argue the days.

Quick quiz

1. What does windows analysis compare in each window?

   1. Costs
   2. Forecast completion at the start and end of the window
   3. Labour hours
   4. Change orders

2. Why does windows analysis capture the claimant's own delays?

   1. It uses only the claimant's events
   2. It uses the updates as they were, which show every party's delays
   3. It ignores the critical path
   4. It removes owner events

3. What must the windows add up to?

   1. The claim amount
   2. The total project delay
   3. The float
   4. Zero

4. In Northgate's W2, how many days were concurrent?

   1. 5
   2. 10
   3. 15
   4. 0

5. What are Northgate's owner, neutral, concurrent and contractor totals?

   1. 18, 14, 10, 18
   2. 52, 0, 0, 8
   3. 60, 0, 0, 0
   4. 14, 18, 10, 18

Show answers and explanations

1. B. The movement of the forecast completion is the delay in that window.
2. B. Contemporaneous updates reflect what actually drove the path.
3. B. Reconciliation to total delay is a key check.
4. B. 10 of the 15 days overlapped the mezzanine redesign.
5. A. Owner 18, neutral 14, concurrent 10, contractor 18.

Key terms

Windows analysis

Analysis of delay period by period, using schedule updates at each window boundary.

Window

A period of the project, usually between accepted updates or milestones.

Recovery

A gain in forecast completion during a window.

Contemporaneous update

A schedule update prepared during the project, at its data date.

Reconciliation

Checking that the delay in all windows adds up to the total delay.

References

1. AACE International, Recommended Practice No. 29R-03, Forensic Schedule Analysis, table of contents and method taxonomy (revision April 25, 2011).
2. Miller Thomson LLP, Ontario Court underlines the evidence and analysis required to establish critical path delays (July 2021).
3. Fenwick Elliott, The SCL Delay and Disruption Protocol: a second edition (November 2017), summarizing the Society of Construction Law Protocol, 2nd edition (February 2017).
4. BRC, Northgate fact sheet (fictional case study, v1, November 4, 2026).

Verified on November 9, 2026.

Next lesson in this trackConcurrent delay in Canadian law