Should I allow the user to err with explanation why, or remove possibilities to err without explanation?

Question

I'm working on a simple design that raises not-so-simple discussion. This is my UI:

User can add new items (country or region, i.e. State/DMA) using the combo above. When using combo at first, it shows all countries and regions. Our system has the following limitation: You can either select countries, or you can select regions inside one country. You can't do both.

There are two approaches:

1. Don't allow the user to make mistakes. Lead him through the UI. This means that if the system sees that user selects only countries, remove regions from combo - we don't allow them anyway (the "Apple" approach).

2. Don't change the content of the combo. Allow the user to select a non-allowed selection (e.g. select a region after he selected a few countries) - and then show a warning with explanation of the limitation and actions to fix the problem (the "Android" approach).

I'll assume the users have a selection in their mind when they approach the UI, and they may not be aware about our limitations. What would be the better approach in this case?

Is there a 3rd way - to not allow user to err but also help him understand the rules?

Answer 1

Avoid the error. It will only lead to frustration and confusion.

Add help text to instruct the users about the system constraints. You could also add an additional highlight if the user does select an illegal selection to help correct (if it is allowed, of course).

Error prevention Even better than good error messages is a careful design which prevents a problem from occurring in the first place. Either eliminate error-prone conditions or check for them and present users with a confirmation option before they commit to the action.

A classic but still relevant: 10 Usability Heuristics for User Interface Design by JAKOB NIELSEN on January 1, 1995