GP was talking about something else though, the 90:90 rule is related to an extremely common planning optimism fallacy around work required to demo, and work required to productise.
It's not just demos though. It's that the final 10% of any project, which largely consists of polishing, implementing feedback, ironing out bugs or edge cases, and finalization and getting to a point where it's "done" can end up taking as much effort as what you needed to complete the first 90% of the project.
Can you elaborate? I am curious. In my line of work, the 80/20 rule is often throw about, that being "to do 80% of the work, you only need 20% of the knowledge." I thought the other reply was talking about the same diminutive axiom, but now I am not sure.
The sibling post gives a good account of the 90:90 challenge.
The last part of any semi-difficult project nearly always takes much longer than the officially difficult “main problem” to solve.
It leads to the last 10% of the deliverables costing at least 90% of the total effort for the project (not the planned amount; the total ad calculated after completion, if that ever occurs)
This seems to endlessly surprise people in tech, but also many other semi-professional project domains (home renovations are a classic)
