Measuring Slow Systems
Some systems reveal themselves quickly; others take their time. The hard part of measuring the slow ones is not the measurement — it is the patience and the bookkeeping required to tell signal from noise.1
The problem with fast proxies
When the outcome we care about is slow, we reach for a faster proxy. Proxies are seductive because they move now. But a proxy is only as good as its correlation with the real outcome, and that correlation is rarely stable.
Goodhart's shadow
Once a proxy becomes a target, people optimize the proxy rather than the outcome.2 The measure stops measuring. The defense is to keep at least one slow, expensive, honest metric that nobody is allowed to game.
Designing the assay
A good assay for a slow system has three properties:
- A stable baseline — you cannot detect drift without it.
- A unit of observation that survives aggregation.
- A pre-registered question, decided before the data arrives.
What to do while you wait
Waiting is not idleness. Instrument the system, write down predictions, and let the slow metric accumulate. The discipline is to not act on the fast proxy before the slow signal has had time to speak.