Interpreting STELLA model diagrams

The basic unit of any STELLA model is the stock.

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Below is a very simple STELLA model. In this model, we are trying to calculate the amount of ozone that forms over some period of time. There are two processes by which the amount of ozone changes:

1. some source of ozone -- in a real model, there would be multiple sources of ozone
2. some ozone sinks -- a sink represents some decrease in the substance being measured, in this case ozone is being destroyed by some unnamed process or processes.