You are the HR Director at General Hospital, and the nursing workforce is yours to manage.

The system you are responsible for runs on exactly seven rules. No hidden assumptions, no trick wording.

  1. There are two kinds of nurses, experienced and inexperienced. Only two.
  2. For months the hospital has held the same number of experienced nurses.
  3. Over that same period, a steady number of experienced nurses have quit each month.
  4. Policy is to hire one inexperienced nurse immediately for every experienced nurse who quits.
  5. Every inexperienced nurse completes a six-month training program and then becomes experienced.
  6. The training is good enough that no inexperienced nurse ever quits before finishing it.
  7. Three months from now the quit rate rises once, permanently. From then on, experienced nurses quit at that higher rate indefinitely.

That is the whole system. Seven rules, two categories.

A real workforce has dozens of moving parts and layers of policy stacked on top of them. This one should be easy.

So: three months from now the quit rate jumps and stays there. What happens to the number of experienced nurses on staff?

Don't overthink it. Sketch the shape, trust your instincts, and write it down before you read any further. The rest of this only works if you have committed to an answer you can be wrong about.

Here is what comes back when I run this with a room of thirty people.

Eight patterns, and the disagreements are not about magnitude. Some people expect oscillation. Some expect a slide that levels off. Some expect a dip and a full recovery. Some expect a decline all the way to zero. Those are different theories about how the system works, and in a real hospital they would produce different staffing decisions, different budgets, different arguments with the CFO.

If thirty thoughtful people cannot agree about a system with seven rules and two variables, consider what happens in a room arguing about a system with two hundred.

Now go run it.


Stop here if you haven't run it yet.


At month three the quit rate rises. The experienced nurse stock declines steadily for exactly six months, and then levels off at a new, lower number.

Most people do not predict that on the first try. In sessions with senior engineers, executives and seasoned managers, almost no one does. Mental simulation runs out sooner than almost anyone expects, and no amount of domain expertise extends it.

The logic, once you can see it:

  • When quitting jumps at month three, hiring jumps immediately to match it. That is the policy, working exactly as designed.
  • Those new hires will not be experienced for six months. So for six months, more people are flowing out of the experienced stock than are flowing into it.
  • Whenever outflow exceeds inflow, a stock falls. That holds for any accumulation, anywhere.
  • At month nine the first wave of new hires finishes training and starts arriving. Inflow matches outflow again, and the level holds at its new lower value.

The system does not oscillate. It does not recover. It settles lower, and it takes exactly six months to get there.

An operational mental model of the HR staffing assumptions

The picture is what makes the argument. Two stocks, three flows, one policy. The inexperienced nurse stock is drawn as a conveyor, which is the modeler's way of saying that everyone who steps on takes exactly six months to come off the other end. That delay is doing most of the work here, and it is nearly invisible when the same seven rules are written out as sentences.

Words are where the ambiguity comes from. We describe one relationship at a time, and we cannot hold two accumulations and a six-month delay in mind at once. What we end up with is a partial map, accurate in pieces and never assembled into something that runs.

If you want experienced nurses to hold steady, the rate at which people become experienced has to match the rate at which they quit. If you want trust in a community to grow, trust has to be built faster than it is lost. We usually end up working on one side of a stock when the answer is sitting on the other.

Which side of the stock am I working on? That question is worth asking about anything you are trying to move: headcount, cash, morale, burnout.

If you want the longer version, I walk through this exercise end to end in about eleven minutes, including what happens to people's guesses once they can see the map.

Watch the full walkthrough

The Orbital Shift Lab is where people practice this on their own work. Members bring a system they are responsible for, map it, and find out where it surprises them. Joining is free.