Two important input controls are radio buttons and drop-down menus. Both of them can be used interchangeably when an input is required from user.
However, if we study about the usability of these controls, it becomes apparent that radio buttons and drop-downs should be used in certain scenarios to make it easier for user to select a given input.
Below are few rules based on this usability study that will help you to decide about one of these controls while designing a form.