A List Apart’s article on Inline Validation is one of the most informative I’ve read in a while — and it’s backed by solid data.
Some useful lessons:
- Inline validation can reduce form completion time by 40%
- Use inline validations where the user doesn’t know if they’ll get it wrong (e.g. is a username available?). Don’t use them if user knows the answer (e.g. their name)
- Validate on blur, not on keypress (it’s distracting, and users can’t multitask)