Apply revision de precios to a public works contract

Read the contract for three things: whether revision was provided for, which formula it names, and from which date and after which qualifying period it bites. Record all three against the contract rather than in a spreadsheet on somebody's machine.

Why: Most of the effort spent on revision de precios is spent on contracts that never carried the entitlement, and the answer takes ten minutes to establish at the start of the job instead of at the first month you want to claim. The base period matters as much as the formula: the same published indices give a different answer from a different base.

Create a series for this contract's formula and add one point per month: the period and the coefficient that formula produced for it. The polynomial split across steel, energy, cement and the rest is worked out from the published indices outside the platform, and the coefficient is what you record here.

Why: Keeping the coefficients as a dated series means the revision on month nine can still be reproduced in year three, when the person who calculated it has left and the published tables have been reissued. A number that exists only inside one certificacion is a number nobody can defend twice.

Apply the coefficient to the amount certified in that period and carry the result as a separate line, alongside the certificacion rather than inside it. The contract rates in the bill stay exactly as awarded.

Why: Revision folded into the unit rates destroys the only comparison anybody has: certified against contract. It also compounds silently, because the following month is revised against rates that were already revised, and nobody discovers it until the final account will not reconcile.

Attach the published tables for the periods you used and the working that turned them into the coefficient, filed against the project and dated. One page per period is enough as long as the numbers on it are the ones you applied.

Why: Published indices get revised after publication, so an audit two years later can look up the same month and get a different figure. The version you used, filed on the day you used it, is the difference between a defensible calculation and a disputed one.

Report the revision recognised to date against the certified value, and carry the coefficient trend forward over the work still to come so the forecast reflects it.

Why: Revision is usually treated as a windfall that turns up in the accounts, which means it is never in the forecast and never in the cash plan. On a three-year contract it is a material share of the turnover, and a job that is not counting on it is also not noticing when a month has been missed.