Sometimes, in order to maintain the flow of a survey, it’s best to keep all questions looking as similar as possible. For a survey where all questions are required, marking each required question simply reiterates what survey-takers already know. This kind of marking stops being emphasis and instead becomes visual “noise” that survey-takers quickly learn to ignore.
In surveys with a mix of required and skippable questions, though, each required question is a possible speed-bump in survey-taking. In these cases, leaving a required question blank stops a survey-taker’s progress and interrupts the flow of taking the survey. Particularly in surveys with a mix of skippable and required questions, emphasizing each required question gives participants a useful visual cue for completing the survey easily.
Changing the format of the question text is an easy and extremely helpful step in noting required questions. Bold is an excellent emphasis choice in these situations because the darker, more prominent font makes the required questions stand out.