A computation is deterministic when the same inputs and the same formula always produce the same output. No sampling, no estimation, no model whose behaviour varies between runs.
For a metric that will be reported to a board or examined by a regulator, this is not a technical preference. It is the difference between a number you can defend and a number you can only assert.
The value can be reproduced on demand, months or years later, from the retained records.
Two people computing independently get the same answer, which removes an entire category of argument.
A change in the output can be attributed to a change in the inputs or a change in the formula, since nothing else can have moved it.
The result can be explained in full rather than characterised. “It is 87 percent because these 4,312 of these 4,957 records met these criteria” is a different kind of statement from “the model estimates approximately 87 percent.”
This is not an argument that estimation is inferior. It is an argument about placement.
Risk quantification is inherently probabilistic. Loss exceedance curves and Monte Carlo simulations produce distributions rather than facts, and that is correct, because the thing being modelled is uncertain.
Threat prediction, anomaly detection and forecasting are all legitimately probabilistic.
The distinction is that these estimate what might happen. Metrics record what did happen and what is currently true. A model belongs on top of a deterministic factual base, taking the facts as input. When the base itself is probabilistic, nothing underneath can be checked.
A measurement layer built on a language model or a similarly non-deterministic process is fluent, useful for investigation, and unsuitable as a record.
Ask the same question twice and the answers may differ in ways nobody can account for. There is no formula to version, no computation to reproduce, and no way to demonstrate that the figure reported last year was produced the same way as this year’s.
That is fine for exploration and unacceptable for the numbers a board is being asked to hold itself to.
From the blog