Should comboboxes/radio groups be marked "required" in a form?

Question

Suppose you have a web form, with some fields being optional and some being required.

One of these fields is a combobox or radio button group, for which a valid default option is pre-selected and it is not possible to "un-set" this field.

In other words, this field is always valid for submission, but potentially not correct, and I can't have users "glossing over" that field as many will with all of the "optional" fields.

Should this field be marked "required"?

Note: I would prefer to keep the default pre-selected option, because this value is the correct value for more than half of the use cases by volume.

Answer 1

It is recommended to select a default. It is also very helpful that you already know the most popular choice, which you would obviously put first in the list and preselect.

Marking mandatory input is common practice, even for a radio selection where a default is selected and where de-selecting is not provided, and even more so if for consistency reasons if there are other input elements in the form. (If this radio selection is the only interaction on the site, you could argue it is not necessary to mark it mandatory.)

Quoting from the NNgroup article Radio Buttons: Select One by Default or Leave All Unselected?

When a choice in a set of radio buttons is known to be the most desired or frequently selected one, it is very helpful to select this by default in the design. Doing this reduces the interaction cost and can save the user time and clicks.

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If you would not want to default users to a value and have them actively think and select, then a dropdown might be a good alternative.