P6 Schedule Comparison: A Practical Guide for Construction Schedulers

Schedule comparison is one of the core skills of construction project controls. Done well, it keeps the project team informed, holds subcontractors accountable, supports owner reporting, and builds the documentation record that protects everyone if a dispute arises.

This guide brings together the practical knowledge a P6 scheduler needs to run a complete, defensible schedule comparison — from setting up the comparison correctly to interpreting the output to producing reports that are actually useful.

Before You Start: File Quality Matters

A comparison is only as good as the files going into it. Before running any comparison, check the basics on both schedule files:

• Data date. Confirm both files have the correct data dates recorded. A file submitted for the March update with a February data date has not been properly updated. In P6, the data date is the PROJECT.last_recalc_date field.
• Recalculation. Confirm both schedules have been recalculated in P6 after the last status update. Float values and CPM dates from a schedule that has not been recalculated after updates are unreliable.
• Project selection. For XER files that contain multiple projects, confirm you are comparing the correct project in each file. Change Inspector shows a project selector when multi-project XER files are loaded.
• File format consistency. XER and XML files of the same schedule are comparable, but there are format differences in how dates, float, and percent complete are stored. Change Inspector normalizes these differences automatically, but be aware that a tool that does not normalize them will produce false positives.

Setting Up the Comparison Correctly

In Change Inspector, load both files and use the Schedule A and Schedule B selectors in the Compare module to designate which is the baseline (older) schedule and which is the update (newer). This matters because the comparison is directional: Added means present in B but not A, Removed means present in A but not B.

For a monthly update review, Schedule A is last month's update and Schedule B is this month's. For a baseline-to-current comparison, Schedule A is the approved baseline and Schedule B is the current update.

For a delay claim analysis, Schedule A might be the last schedule before the delay event and Schedule B the first schedule after — or any two files in the project's update history that bracket the period of interest.

The Comparison Workflow — In Order

A complete comparison review follows a consistent sequence:

1. Schedule Options first. Before looking at any activity data, check the Schedule Options tab for differences in P6 calculation settings. A change in Retained Logic or float type will affect float and CPM dates across the entire schedule. If settings changed, document why before interpreting activity-level changes.

2. Overview — macro picture. How many activities were added, removed, changed? Is the scheduled finish date moving? Is the critical activity count changing significantly? These summary numbers frame everything that follows.

3. Added and removed activities. Review additions and deletions carefully. Added activities should have proper logic connections. Removed activities need an explanation.

4. Changed activities — critical and near-critical first. Filter the Activities tab to show Changed activities only. Sort or filter further to prioritize critical activities (total float ≤ 0) and then near-critical (total float under 20 days). Review date changes, duration changes, and status changes on these activities before anything else.

5. Relationships tab. Review all logic changes. Flag any removed predecessor on a critical activity, any new relationship that appears to constrain activities artificially, and any lag changes.

6. Float consumption. If you have two or more files loaded, check the Float Consumption analysis in the Health Check module. Note the top float consumers and flag any activity that went critical between the two schedules.

7. Gantt Diff. Sort by Delay for a visual picture of the most impacted activities. This is the view most useful for communicating findings to non-schedulers.

8. Export. Export the comparison results to Excel for the official record. Attach to the monthly report submission or project controls file.

Common Pitfalls in Schedule Comparison

Not checking schedule options

As noted above: if P6 calculation settings changed between the two files, float and date changes in the Activities tab may reflect that change rather than genuine schedule updates. Always check the Schedule Options tab first.

Treating all changes as equally significant

A comparison report on a large schedule may show hundreds of changes. Most of them are minor — float values shifting slightly, remaining durations decreasing as work progresses, status codes updating. The discipline is in knowing which changes are significant and which are normal update activity. Focus on critical and near-critical activities, milestone movements, logic changes, and large duration changes first.

Ignoring the relationships tab

The activities tab shows what changed. The relationships tab shows why the critical path may have shifted. Schedulers who review only the activities tab will miss the most consequential category of changes.

Not running the health check

A comparison tells you what changed. A DCMA health check tells you whether the schedule is well-built. They answer different questions and both are necessary. Running the health check at the same time as the comparison — especially when reviewing a schedule submitted by a subcontractor — gives you a complete picture of schedule quality and specific changes.

Producing Comparison Reports That Hold Up

Whether you are submitting to an owner, sharing with a subcontractor, or building a claims file, the comparison report needs to be:

• Organized — grouped by change type (added, removed, changed) and by WBS or significance
• Attributable — showing which file is Schedule A, which is Schedule B, what the data dates are, and when the comparison was run
• Complete — covering activities, dates, relationships, and schedule options — not just a subset
• Ready to use — formatted for the audience, not requiring manual reformatting before it can be shared

Change Inspector's Excel export produces a color-coded, multi-tab report covering all comparison dimensions in a format ready for owner submission. The Schedule Log export adds a chronological record of all schedule files and their metadata.

The combination of a thorough comparison report and a well-maintained Schedule Log is the documentation foundation that supports both routine project reporting and, if necessary, a formal delay claim analysis.