Error messages. Not the critical ones that require admin intervention, but those informative bits of text telling you that you missed a field on a form.
Chances are, you won't see a 100% success rate when people fill in a form on your site. The more complex the form, the more likely it is that people won't reach the end.
Forms should be easy to follow, and easy to recover from in the event of missing or invalid data. Here are some tips to help users understand what went wrong, and not just leave them hanging.
Give the Expected Format
If you're asking for a phone number, show what the expected format is. It's really frustrating to enter a phone number only to be told it's in the wrong format, but without being told where you went wrong.
Remember, the Internet is international - so unless you actually block people from other countries from visiting, you can't assume that everyone will have a US-style phone number. Coming from the UK, this has prevented me from signing up to certain websites that don't clearly state "for US residents only" and appear to be international.
Make Dates Easy
Following on from phone number formats, different people will be familiar with different date formats. I'm used to entering the date in DD/MM/YYYY format - putting the month first is not what I'm familiar with.
Whichever format you choose, either include a brief note to show the expected format, or use month names instead of numbers - this will make it very clear how to enter the date.
It's also worthwhile to use a date picker - this makes it very easy to select a date without having to type it in.
Some Fields Were Missing
Could you prevent this error by clearly indicating which fields are required? And if some fields were missing, it's vital to tell the user which ones.
How Many Fields Are Required?
Is it necessary to have a phone number? Depending on the site, you might need one. But if you're trying to force a phone number to be entered when it isn't absolutely necessary, many people won't fill it in at all. Some people may enter a fake number just to get the form to submit.
What else could you do to help inform users of form submission problems?
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