In UI design, a user model is how the user thinks something operates. This is my user model for the shower in my apartment.
- I turn the faucets to a desired temperature.
- The settings are transferred by mechanical means to the basement.
- Sensors there determine the position of the controls. These are converted into verbal instructions and communicated by telephone to an old man, who actually adjusts the amount of cold and warm water.
- The old man is deaf, grumpy, and frequently away or drunk, we don’t know.
- He’s of mixed Italian and Serbo-Croat descent, and sometimes he hears “cold” as caldo (Italian ‘hot’), or “hot” as Serbo-Croat hladan ‘cold’.
- Sometimes salesmen call; he takes whatever they say as more commands and changes the water setting. Or maybe it’s just sheer cussedness.
- There is a range of settings that produces a warm, comfortable shower. These settings are marked “DO NOT USE, FIRING OFFENSE” and he strictly avoids them. However, he can sometimes be coaxed into this range by confusing him with frequent, contradictory orders.
This user model explains and predicts the shower behavior quite reliably.