Bathtub Test

Something keeps piling up or running down despite effort. Name the quantity that persists, what enters it, and what leaves it over the same period.

What is stored? What flows in? What flows out?

Treat an accumulating quantity like water in a tub. Separate the current amount from rates that add and remove it, then compare those rates using a common time unit. This can explain an open dynamic; you do not need to invent a closed loop.

Try this when…

I work on it daily, but the pile keeps growing.

We use it faster than it gets replaced.

The number is high, but I do not know why.

We keep changing effort without checking the rates.

What you can use it for

Backlogs

Compare arrivals, completions, and rework that returns items to the pile.

Shared supplies

Distinguish a one-time stock increase from ongoing replenishment.

Creative production

Track unfinished work as a stock with real completion and abandonment flows.