Three quantities for the same wall
Ask three people how much of a wall exists and you get three answers.
The bill of quantities says one thing, priced at tender from a design that has since moved. The drawing says another, because it is the current revision and the design changed. The survey says a third, because that is what is physically there, including what was built slightly differently.
None of them is wrong. They are answers to three different questions
: what was priced, what should exist, and what does exist.
Where the three quietly diverge
The failure isn't that the three differ. It is that different documents silently pick different ones.
Progress is usually reported against the bill, because that is what the baseline was built from. The site reports against the drawing, because that is what they are building. The quantity surveyor measures what exists, because payment depends on it. Three documents, three denominators, and a percentage that can't be compared across them.
It gets worse with variations. A change increases the drawing quantity without touching the bill. Progress against the bill now rises when nothing has happened — the denominator stayed still while the numerator grew
— or falls for the same reason inverted. This is the mechanism behind a curve that moves for no visible reason, and it is worth recognising because it looks exactly like a data error.
Choosing the denominator
The rule that works is simple and rarely written down: report progress against the current approved quantity, and re-baseline the denominator when it changes, not silently.
Which means every variation has a second consequence nobody enjoys. The scope changed, so the total changed, so last month's percentage was against a different total. Either you restate it or you note it.
Doing neither produces a series that looks continuous and isn't.
Cost & Cash Week 4 covers how the bill relates to the schedule. What this week adds is that the relationship doesn't hold still, and the report has to say when it moved.
Who owns the denominator
There is an ownership question underneath this and it is usually unanswered, which is why the three sources drift.
The bill belongs to the commercial team. The drawing belongs to engineering. The measurement belongs to the quantity surveyor. The denominator used in the progress report belongs to nobody in particular, and so it is whatever the spreadsheet was built with, by somebody who has since left.
Naming an owner for it does more than it sounds. It means somebody is responsible for updating the total when scope changes, and for saying out loud that last month's percentage was against a different number. Without that, the update happens silently or not at all.
If the orders are already placed
If the project has not started, this is one decision recorded in the procedure. Progress is measured against a stated source, variations update it, and the update is visible in the report.
Inherited: take three documents from last month — the progress report, the payment application and the site's own tracker — and find one activity in all three. If the totals differ, you have found the seam. It is usually not worth changing the system for it. It is always worth knowing which document uses which, because that is the difference you will be asked to explain.
System design
The three sources and what each is entitled to answer. The column that matters is the last one, because that is where the disagreements surface.
| Record | Produced by | Required quality | Verified against | Feeds |
|---|---|---|---|---|
| Bill quantity | Estimating | Frozen, with variations tracked separately | Contract | Baseline, payment |
| Drawing quantity | Engineering | From the current revision, dated | Revision register | Site, procurement |
| Measured quantity | Quantity surveyor | Measured under the contract rules | Site measurement | Valuation |
| Denominator in use | Project controls | Stated, and restated visibly when scope moves | Change register | Every progress percentage |
From scratch, the denominator is stated once in the procedure. Inherited, the useful step is to find out which of the three each existing document already uses.
Practical insight
Take one activity that has been through a variation. Write down its quantity as the bill has it, as the current drawing has it, and as measured.
Then find the percentage your last report gave for it and work out which of the three was the denominator. On most projects it takes ten minutes and the answer surprises somebody.
Key takeaways
- Bill, drawing and survey answer three questions: what was priced, what should exist, what does exist.
- The failure isn't that they differ. It is that different documents pick different ones without saying so.
- A variation moves the drawing quantity without moving the bill, and the percentage shifts with nothing built.
- Report against a stated source and update it visibly when scope changes.
- A restated denominator makes a series discontinuous. Say so rather than smoothing it.
- Inherited systems rarely need changing here. They need somebody who knows which document uses which.
Records born here. Quantity source statement · the re-baselined denominator after each variation.
What is coming next
Quantities describe what was built. The other half of every productivity figure is what it took, and that number has a different problem.
Next week: hours and plant — why the total is right and the allocation isn't.
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