How to Read a Schedule Comparison Report

A schedule comparison report tells you what changed between two versions of a project schedule. Done well, it is one of the most useful tools in construction project controls. Done poorly — or read without knowing what to look for — it is a list of changes with no clear signal about what actually matters.

This post walks through how to interpret a schedule comparison report, where to focus first, and how to turn the output into action.

Start With the Summary

Before diving into individual activity changes, look at the overall picture first.

A well-structured comparison report starts with a summary: how many activities were added, how many removed, how many changed. Change Inspector's Overview tab shows this at a glance along with key metrics from each schedule — data date, scheduled finish, number of activities, critical activity count — so you can see at the project level whether the schedule is moving in the right direction.

What you are looking for at the summary level:

• Is the data date current? A schedule submitted with last month's data date has not been properly updated.
• Is the scheduled finish moving? Even a small shift forward over multiple update cycles compounds quickly.
• Are critical activity counts changing significantly? A sudden jump in critical activities often signals a logic change or a constraint being added.

Review Added and Removed Activities

Added and removed activities are the most visible category of change and deserve careful attention.

Added activities may be legitimate — scope was added, work was broken into more detail, a new phase was incorporated. But they may also indicate the scheduler is adding float to the schedule by fragmenting existing activities, or inserting placeholder activities that pad the timeline.

Questions to ask about added activities:

• Are they properly connected to the network with predecessors and successors?
• Do they have reasonable durations?
• Is the addition supported by a scope change or approved schedule revision?

Removed activities are often more concerning. An activity disappearing from the schedule between updates needs an explanation. Was it completed and removed? Merged into another activity? Or was it deleted to hide a problem?

In Change Inspector, added activities appear with a green Added badge, removed activities with a red Removed badge. You can filter the Activities tab to show only added or only removed activities for focused review.

Focus on Changed Activities — But Filter Strategically

A large schedule update might show hundreds of changed activities. Not all changes are equally significant. Here is how to prioritize:

Date changes on critical activities first. Any change to early start, early finish, or float on a critical activity (total float ≤ 0) has immediate implications for project completion. Look at these before anything else.

Milestone date changes. Contractual milestones shifting — even by a few days — are significant. Check the Schedule Dates tab specifically for milestone movement.

Status changes. An activity changing from In Progress to Not Started is a red flag. So is a completed activity reverting to In Progress. These indicate status update errors or deliberate manipulation.

Float changes on near-critical activities. Activities that were comfortably non-critical but are now showing reduced float are moving toward the critical path. This is a useful early warning signal before a problem becomes acute.

Duration increases on incomplete activities. If an in-progress or not-started activity has had its planned duration increased, that is a forecast change — the scheduler is predicting it will take longer than originally planned.

Logic Changes Are the Critical Reads

The Relationships tab of a comparison report is where experienced schedulers look most carefully — because logic changes are the ones most likely to have been made quietly and to have significant schedule consequences.

For every changed relationship, check:

• What was removed? A removed predecessor may mean an activity is now floating free of its constraint. A removed successor may mean downstream activities are no longer anchored to the work that precedes them.
• What was added? New relationships may be legitimate sequencing additions or may be artificially constraining activities to prevent float from accumulating.
• What lags changed? A lag that increased between two updates extends the time between the predecessor and successor — effectively inserting time into the schedule without adding a visible activity.

Pay particular attention to logic changes on the critical path. A relationship change that is not on the critical path today may be significant if it affects activities that are near-critical or become critical in a future update.

Check the Schedule Options Tab

Before drawing conclusions about why float or dates changed, check whether the schedule calculation settings changed between the two updates.

If Retained Logic changed to Progress Override, float values across in-progress activities will look different even if no actual work sequences changed. If the critical float threshold changed, the count of critical activities will shift. If the data date moved differently than expected, date comparisons may be misleading.

In Change Inspector, the Schedule Options tab flags any differences in P6 calculation settings at the top with a badge count. Check this tab first whenever the Activities tab shows widespread, unexpected float or date changes.

Gantt Diff — Visualize the Impact

After reviewing the data, use the Gantt Diff view to visualize the changes. Sorting by Delay shows you which activities have moved the most, with a badge showing the number of days of delay relative to the previous update. Sorting by Float puts critical and near-critical activities at the top.

The Gantt Diff is particularly useful for communicating changes to stakeholders who are not scheduler-fluent — a visual timeline of what shifted, sorted by impact, is far more accessible than a data table.

What to Do With the Findings

A comparison report is only useful if it leads to action. Once you have reviewed the results:

1. Document significant findings — note which activities changed and whether the changes were expected and approved or require explanation.
2. Flag issues for follow-up — activities whose changes are unexplained, unauthorized, or concerning should be raised with the responsible party.
3. Export and attach — the formatted Excel export is your record. Attach it to your monthly report, your owner submission, or your project controls file.
4. Build the audit trail — add the current schedule file to the Schedule Log with a note documenting the update. The running log is what makes the comparison history useful months later.