Pattern 71: Mandatory fields
AKA:

You know that you should avoid pre-emption in interface design. But you need to capture some information about users, perhaps as part of a workflow application. INTERNATIONALIZATION (52) dictates that you will often not know how users normally express themselves or describe themselves. You can predict that many of then will be impatient. You are aware of the need for ACCEPTABLE WORDING (50).

The problem is to capture the essential information without trying the patience of the user or upsetting her sensibilities.

Therefore

Make the absolute minimum number of fields mandatory. Give the user an Other option wherever possible. Always indicate visually which fields are mandatory. Use a star to do this and change the background colour of the mandatory fields. Explain any validation rules to the user before she does the typing. In case of validation errors present the form again.

Avoid PARANOID SECURITY (74) checks.

Many web sites offend in the respect. It is a common experience to fill in a form only to find that the site rejects the input because

1. fields have been left blank, or
2. entries have not passed the site’s validation.

Many users will give up and go elsewhere, rather than re-enter their particulars.

If you are validating text explain the rules so that the user has a chance of getting it right first time. For example, explain that there are to be no hyphens in telephone numbers, if this is what you want.
Tidwell’s FORM pattern is related to this pattern.

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