Every month the scheduler statuses the CPM schedule and the software recalculates total float for each activity. The update report usually shows where the critical path stands on that data date. A second number comes from comparing updates: how many work days of float the longest path lost since the last one. Logged month after month, that figure tells a project executive how fast the job is using its float and how many updates are left before the forecast finish moves past the contract date.
The definitions
The Government Accountability Office’s Schedule Assessment Guide defines total float as the time an activity can be delayed or extended before the delay affects the program’s finish date. It can be positive, zero or negative. Negative float is the time that has to be recovered to hold a constrained date, and GAO treats any activity with zero or negative total float as critical.
Free float is the part of total float an activity can use before it delays its immediate successor. An activity can carry 2 days of total float and no free float, in which case a 2-day slip leaves the finish date alone and still pushes the next crew’s start.
GAO calls activities whose float sits within a narrow range of the critical path near-critical, because a small delay makes them critical. The guide gives one example threshold, monitoring activities with 5 days or less of total float, and says the threshold should fit the length of the program.
On reporting, the guide says: “Periodic reports should routinely report the amount of float consumed in a period and remaining on the critical and near-critical paths.”
What a federal schedule spec requires
The Army Corps of Engineers’ guide specification for project schedules, UFGS 01 32 01.00 10, turns those ideas into contract language. The version posted by USACE requires a schedule status report at least monthly. The last activity, End Project, carries a late finish constraint at the contract completion date, so a forecast finish past that date shows up as negative float.
Each periodic update comes with a narrative report. It has to describe the activities on the 2 most critical paths where total float is 20 work days or less, along with problem areas and corrective actions. The update also includes a total float report that lists every incomplete activity in ascending order of total float.
Two clauses decide who gets the float. Section 3.10 says float available in the schedule is not for the exclusive use of either the Government or the contractor. Section 3.7.1 says a request for a time extension must show that the contractor used all the float available for the affected work.
The measure
Five numbers from each update are enough:
- Work days from the data date to the contract completion date.
- Total float on the longest path, read at the End Project activity or the last activity before it.
- Total float on the second most critical path.
- Float used this period, the prior update’s longest-path float minus this update’s.
- Float as a percent of work days remaining, item 2 divided by item 1.
Keep the units in work days on the project calendar. GAO says all float should be in the same units and notes that nonworking periods on a calendar are not float.
A worked example
The figures below are an example with round numbers. A general contractor is building a mid-rise office building with 330 work days from the baseline data date to contract completion. The baseline shows an early finish with 30 work days of total float on the longest path and 35 on the second path.
The longest path has used 26 of its 30 days of float in eight updates
Total float in work days at each monthly schedule update, example office building
| Update | Work days to contract completion | Longest path | Second path | Float used this period | Float as % of days remaining |
|---|---|---|---|---|---|
| 0 | 330 | 30 | 35 | 0 | 9.1% |
| 1 | 309 | 28 | 33 | 2 | 9.1% |
| 2 | 288 | 27 | 30 | 1 | 9.4% |
| 3 | 267 | 22 | 28 | 5 | 8.2% |
| 4 | 246 | 21 | 24 | 1 | 8.5% |
| 5 | 225 | 15 | 22 | 6 | 6.7% |
| 6 | 204 | 12 | 19 | 3 | 5.9% |
| 7 | 183 | 7 | 15 | 5 | 3.8% |
| 8 | 162 | 4 | 11 | 3 | 2.5% |
After eight updates the longest path has 4 work days of float left and 162 work days to go. The job has used 26 days in eight months, an average of 3.25 days per update, and 3.7 days per update over the last three. If the average holds, longest-path float turns negative by the tenth update, with about 120 work days left before contract completion.
The percent column adjusts for the shrinking job. Float held near 9% of the remaining duration through update 4, then fell below 7% at update 5 and to 2.5% at update 8.
The longest path dropped to 20 work days or less at update 5, and the second path followed at update 6. On a contract written like the USACE spec, both paths now belong in every narrative report.
Float used by cause
Attach a cause and a responsible party to each period’s loss, the same way a change order log groups changes by cause. The driving activity is the one on the longest path whose late start or longer duration took the float.
| Update | Driving activity | Cause | Responsible party | Work days of float used |
|---|---|---|---|---|
| 1 | Excavation and shoring | Rock not shown in the geotechnical report | Site condition | 2 |
| 2 | Foundation walls | Rain days above the baseline allowance | Weather | 1 |
| 3 | Structural steel erection | Late steel delivery | Supplier | 5 |
| 4 | Metal deck and slab on deck | Crew shortage | Subcontractor | 1 |
| 5 | Elevator shop drawings | Resubmittal after design comments | Design team | 6 |
| 6 | Roofing | Rain days above the baseline allowance | Weather | 3 |
| 7 | Curtain wall | Crew shortage | Subcontractor | 5 |
| 8 | Switchgear | Delivery date pushed by the manufacturer | Supplier | 3 |
Suppliers account for 8 of the 26 days, subcontractor crews for 6, the design team for 6, weather beyond the baseline allowance for 4 and a site condition for 2. The 6 days from the elevator resubmittal and the 2 from rock are the entries a project manager would document for a possible time extension. Under a clause like USACE section 3.7.1, a request has to show that all available float on the affected work was used, so the log of when and why float was used becomes the evidence.
Reading changes in float
GAO lists several reasons a float value can be wrong. Missing or incomplete logic, unrealistic durations, unstatused work and date constraints all distort it. GAO says significant changes in float can mean a logic link was broken or an out-of-sequence activity was completed.
High float gets the same scrutiny. GAO says unreasonable amounts of total float usually come from missing logic. The guide cites a Federal Aviation Administration schedule in which one testing activity showed 1,280 days of float.
GAO also says float should not be treated as schedule contingency, because it is shared along a sequence of activities and any activity on that sequence can use it.
This log pairs with the SPI and CPI calculation from pay applications. SPI compares earned value with planned value in dollars across the whole job. Float on the longest path shows whether the work that is behind is the work that sets the finish date.
How to start on your jobs
Open the last three updates on one active job. Read total float at the finish milestone and on the second most critical path in each, and enter the five numbers above in a spreadsheet row per update. At the next update, add the row along with the driving activity and the cause of any float lost.