Anyone designing a form will eventually come across the problem of: 1) when to validate the form data which the user has entered, and 2) how to provide the user feedback on fields that don’t meet the validation criteria. Many approaches have been tried for the second problem, but I think the first problem—when to validate the data and provide feedback—could use more consideration.

Suppose you’re designing a form containing a text box in which you will ask the user to enter a mobile phone number. For a variety of reasons, you’ve decided that you want to perform some initial validation of the number to minimize the chance of later problems. There’s no easy way to ensure, on the spot, that the number is, in fact, that of a real mobile phone, but you can at least ascertain via some regular expression that the text looks like a valid phone number. (Er, have fun with that, especially if you want to support international numbers.) Your regex is quick to evaluate, but you need to decide when to evaluate it and do something with the result.

Here are some choices for when to validate the phone number, along with some pros and cons:

When the user types a key.



Pros: The user will get the feedback as quickly as possible, and so they should be able to correct the problem immediately. Because they’re looking at the phone number field, feedback positioned next to the field will be noticeable. Because the user is thinking about the phone number, they don’t need much prompting to spot the problem. The keyboard focus is probably already in exactly the right place to fix the problem.



Cons: It can be incredibly irritating to use a program which instantly complains about a problem which you yourself can tell you’ve made. If you’re reaching for the “1” key, and see you’ve accidentally hit the “q” key, you can see for yourself you’ve made a mistake. You don’t need the additional shame of being yelled at. A program that validates on a key press can act like the jerk in the car behind you who honks their horn the instant the traffic light turns green.



There are also plenty of situations in which such error feedback is completely premature. Suppose the user is pasting in a phone number they’ve copied from their address book or some other location, and the pasted text contains some extra characters. The user can see the extra characters, and would be perfectly happy to remove them if only the program wouldn’t yell so loud.

When the field loses focus (e.g., the user has moved the keyboard focus to the next field).



Pros: The user has had a chance to get the field text into the state they desire, so any feedback at this point is likely more warranted. Perhaps the user may not have noticed that they’re missing a digit from the phone number.



Cons: The user’s attention has already begun to move on, so the error feedback may need to be more prominent. Also, while this technique is less hasty than the one above, the error feedback may still preempt the user’s own ability to recognize the problem. The user has to manually move the keyboard focus back to the phone number field.

When the user attempts to save the form.



Pros: By pressing a commit button like Save, the user has indicated that they think they’re done entering data. If there’s an error at this point, they haven’t noticed it, so the feedback will be timely.



Cons: By this point, the user’s attention may be far from the phone number field, so the feedback needs to be quite prominent. It may need to be supplemented by overall form validation feedback near the commit button or at the top of the page. Once the user sees that overall feedback, it may take a moment for them to visually reacquire the problematic phone number field, diagnose the problem, and get the insertion point to the point where they can correct it.

All these methods have their place, although in a situation like this, I personally prefer to defer giving validation feedback to the point where the user is attempting to save the form (#3 above). I’d rather give the user a reasonable chance to fix any errors, and have observed countless usability studies in which they have done so. I believe this leaves the user feeling in control, and to me this is worth the disadvantages listed above for this approach.

Still, there’s one common downside to this approach: applications that validate on Save tend to keep the error feedback visible until the user tries to save again. I find this annoying; it’s like the program continues to scold me even after I’ve admitted I was wrong and atoned for my mistake. Once I see the problem and have fixed it, I wish I could get credit for fixing it right away.

Asymmetric validation feedback triggers

There’s no reason the triggers for showing and hiding the validation feedback need to be symmetrical. I’ve been working on the idea of asymmetrical validation triggers: validate on save to show validation feedback and validate on keypress hide the feedback. In this scenario, the UI shows validation feedback (validation message, change field background color, etc.) if the user attempts to save the form with something that doesn’t appear to be a phone number. But as soon as the user types the key that fixes the problem, the validation feedback goes away.

The user is typing…

They mistype. The UI doesn’t complain; maybe the user will fix the error.



The user tabs away with the error uncorrected. Still no complaint.

They try to save the form. NOW validation feedback appears (with an explanation nearby).

The user clicks to correct the error…

As soon as they type that fixes the error, the validation feedback goes away.



In other words, the UI is slow to complain, and quick to forgive.

I think this is generally the proper conversational posture for a program to take regarding the validation of user input. Most programs treat validation feedback as if it were the user’s fault, when to me this feels like almost exactly the wrong spirit in which the view the situation. Data validation is a sign of program weakness — a sign that it’s still too hard to design a program that can process input flexibly and resiliently.

If I fill out a paper form that asks for my home and work phone numbers, I can fill in the first phone number and write the word “SAME” for the second phone number, and any human would process that correctly. I can even cross out the field label “Work” and handwrite “Cell” and write in my cell number, and again this is meaningful to a human reader. I can make all kinds of minor errors, and still my input can be interpreted.

That is a breathtaking level of input flexibility which no UI today can match. And so when a form has attempted to recognize user input, and failed to do so, the appropriate stance should be not, “Invalid phone number”, but rather, “I’m really sorry, I’m not smart or sophisticated enough to understand you. Could you please help me by making this more recognizable as a phone number so that I, a mere program, can process it for you?”

I’m not saying that’s what the program should really say, but that’s a useful mindset to have when approaching the situation as a design problem. Such thinking led to the asymmetric validation triggers above, and perhaps more thought along these lines will lead to further refinements to this very common aspect of user experience design.