Monday, 12 November 2001
The Bao Pao (Chinese steamed roll) will be ready soon. I just have to wait a few minutes. I step back and wait.

The Humane Interface
My mind starts wandering, as usual. Jef Raskin ( has given me quite some food for thought with his book The Humane Interface ( His book offers sound theoretical grounds. What makes a user interface a good user interface? His explanation of the locus of attention ( in particular tickled my brain. How does one apply this theory?

Lost in thought I stare down the Gouda shopping street. There is something peculiar about the view but I fail to see what it is.

A little way away someone is looking at me. My mind detaches itself from the world of thoughts and focuses on reality. The image gets clearer. The face comes into focus. Finally I recognise a fellow Tai Chi pupil. Amazing how tough it is to recognise a familiar face in a different context, obstructed by wandering thoughts.

And here it is: a practical example of the locus of attention.

  1. My mind was on the Bao Pao, but started wandering.
  2. The Chinese roll faded into the background and my locus of attention was on the ... locus of attention.
  3. It took quite some effort to notice something that was outside my locus of attention, even though it was in plain sight.
The last bullet is exactly the core of Jef Raskin's point. Users are lost in thought, only see the few square millimetres of screen estate they are working on at that very moment. All other things pass unnoticed.

So, what does one do when a user types text in a numeric field?

  • Display a hackneyed pop-up? No, completely out of the question.
  • Beep and simply refuse non-numeric data? No, too unfriendly. A user must be able to see what is wrong.
  • Display a message in the final result? No, that will escape unnoticed.
  • In a status field? No, just as easy to miss.
  • Straight after the field? No, even that will not do.
The locus of attention is on the field, so the field itself is the perfect location for an error indication. An example screen in Dutch:

A red background suffices. The last typed character is the cause of the error. An error message is obsolete. Simple and very effective.

After a short conversation with my fellow Tai Chi pupil my stomach starts to rumble. The Bao Pao demands my full attention.

Till next week,

Thanks to Antoinette Coetzee for The Humane Interface