For the form items below, which interface control is the best option? A single checkbox or yes/no radio button?
Read EWLS advice letter? Checked images / report on PACS / CRIS? Pacemaker? Unable to localise stone
It would be helpful to get more context around the question and what the user is trying to accomplish. Checkboxes can work and can probably be faster for the user to fill out the form since there will be fewer options to visually inspect. If you are going to go with checkboxes you might need to add clarifying text. e.g. What does "pacemaker" mean? Does it mean a patient has a pacemaker/pacemaker was checked? Just so that when the user checks the options they know what it actually means. Keeping keywords at the beginning of the checkbox would also be good e.g.
My rules of thumb:
If you need to make sure the user actually did consider all available options, you need radio buttons (or checkboxes that behave like):
Did you read the entire contract?............[ ] Yes [ ] No
Twice?.......................................[ ] Yes [ ] No
If this does not apply: Checkbox if the opposite is obvious (usually: label mentions an action, if unchecked that action is not taken, wiht no other side effects).
Otherwise, radio buttons to describe the alternatives.
[x] Include Lunch
( ) Indian
Radio buttons are better in the vast majority of (if not all) cases.
The first part of peterchen's answer is exactly right: If you want to ensure that a choice is addressed by the user, you must use radio buttons. (You could also use a drop-down box, if you are looking at options other than the two in your question.)
More specifically, you must use sets of radio buttons with no default selection, and the problem with checkboxes is precisely that they always have a default selection.
...and you should always want to ensure that a choice is addressed by the user.
This falls under the larger basic principle of validating user input.
(The only exception to this might be if you are designing a detailed settings menu and where you expect that most users will not look at all of the choices in one sitting, and where you expect users to return every so often. Then users can address, on later visits, any choices that they missed on earlier visits. Even here, though, having no default selections helps. In this case it can make it easier for users themselves to identify which settings they have addressed before and which ones they may yet want to try changing.)
A radio should be used where there is a question/choice with more than one option and those options need to be mutually exclusive, e.g:
Can we use your email address for marketing?
( ) Yes
( ) No
Whereas the purpose of a checkbox is for a question/choice where there is only one answer, or multiple choices which are not mutually exclusive, e.g:
[ ] Tick the box if we can use your email address for marketing
I am interested in
[ ] Music
[ ] Art
[ ] Films