# “Stock” or “Memory” Variables

So far, we have considered only quite ephemeral “for-time” Variables whose Level at any point in time is dictated entirely by other Variables:

Average temperature of Irish Sea ~~~

Incoming solar radiation on to Irish Sea ~~~

… but this suggests that as soon as the sun stops shining, the temperature drops correspondingly. But a “for-time” Variable like this is called in System Dynamics23 a “stock” Variable, which means it behaves something like this:

Average temperature of Irish Sea ~~~

Average temperature of Irish Sea ~~~

Incoming solar radiation on to Irish Sea ~~~

This arrow from the Variable to itself is assumed by default to maintain the Level in the previous instant.

An eternal Variable whose Level never changes ~~~

An eternal Variable whose Level never changes ~~~

Whereas in this case, a teacher’s skill is supposed to naturally decline from instant to instant, which might be a good reason to have some top-up training from time to time:

Teacher skill level ~~~

(!-)Teacher skill level ~~~

Top-up training ^^^

In this final example, we learn that the client’s Level of animal phobia is likely to increase over time untreated, but if guided exposure to the feared animal is carried out (say, each week), then the Level of phobia will decrease from the previous time-point.

Client's animal phobia  ~~~

(+)Client's animal phobia  ~~~

(-)Weekly guided exposure to feared animal ^^^^

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