This prompts me to have a little chat about the difference between stocks and flows.

A threadette on system dynamics. With a picture of a bath for good measure. https://twitter.com/allisonpearson/status/1329222439449075713

Think of your bath. Here's one I prepared earlier for one of my lectures.

Except let's think about patients in hospital. That's the water level in the bath.

The water flowing in from the tap in is patients admitted, and water going down the plughole is patients discharged.

The important thing is not to compare stocks with flows or flows with stocks. Otherwise you get into a bit of a mess.

Let's look at the data (if you want to follow along at home it's here https://coronavirus.data.gov.uk/details/healthcare)

Patients admitted to hospital. This is a flow into the system.

There are also patients being discharged. That's a flow out of the system. There's no graph for that (which is a bit of a shame, as some people can unwittingly become confused).

So - how do we know that the flow in (admissions) is greater than the flow out (discharged)?

We can look at the number of patients in hospital. That's a stock.

And it's going up. That means that more people are being admitted than being discharged.

This is a bad thing.

Here's the data for the last few days. This puts pressure on the NHS, and has its own indirect effects - elective surgery being cancelled, pressure on beds - and in particular staff - nurses and doctors - if you have an accident, that sort of thing.

So, don't get confused between stocks and flows.

And don't let that bath overflow, or you'll have to clear up the mess that ensues.