Optional question with one selection from multiple options

Question

I have a form where I have many questions that:

1. Can be left unanswered
2. Have short list of options that should be all visible
3. Only one of the options can be selected at a time

The current solution is to use radio-button for the alternatives and it works well except that if a user accidentally selects an option for a question he doesn't intend to answer he can't undo the selection. What would be the best solution for such scenario? Should I just add a button to clear the answer?

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In this answer it's said that "You're not supposed to leave radio buttons blank", but besides drop-down list I haven't come up any better control for the case. Forcing group of check-boxes to only accept one them selected seems bad way to go. Is there another alternative solution that I've missed?

Drop-down list won't work as in this case I wan't the user to able to always see all the possible options.

I wan't to emphasize that I need be able to tell if user has answered the question or not. And it needs to be possible to leave a question unanswered. Forcing the user to answer each question and adding an option for "Not answered" (and maybe making it default option) could be one way to go, but I'd rather leave the unanswered question truly unanswered.

"Based on the user input I've received the case where user accidentally chooses one of the options and wants to remove his selection is quite rare." There should still be some way for users to make a question unanswered again.

Answer 1

In the answer you cited:

You catch this in validation (preferably in page). This lets you make sure the user has filled in the field, rather than assuming the default, which can be very important.

You stressed that you "need be able to tell if user has answered the question or not" and that "it needs to be possible to leave a question unanswered," but to what end. I'm trying to understand your problem, and am providing a few solutions.

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To provide additional clarity:
When the user presses Submit (or whatever the trigger is for the next step), verify that the user has selected an option.

^{download bmml source – Wireframes created with Balsamiq Mockups}

An Alternative:
Pre-check "no answer" for every question, which frees you up from checking that each set of radio buttons has an option selected. What is your aversion to this? Are you checking to see if people actually saw the questions?

If you're worried about distinguishing between "I don't want to divulge" and "I didn't see this question/skipped it," here are two options. The right image has a disabled pre-selected option (so it can't be reused).

^{download bmml source}

Another Alternative:
"Many questions" is pretty vague? If you can group questions, you can check check on individual sets. If some questions do require answers and others don't, you can break up the flow into several sections with "Next."

Yet Another Alternative:
You could have "clear answers" near the submit button. I would just discourage you from putting one on each question because it'll get more cluttered and could confuse.

The counterpoint though is that they can't clear individual answers. Which makes breaking it into manageable chunks a more attractive option.