Surveys and interviews differ in which important way?

Interviews are in person, therefore there may be interviewer and interviewee bias in regard to the questions being asked.  Only so many interviews can take place during any given period of time.  Surveys eliminate interviewer and interviewee biases.  And surveys can reach a large majority of people.  However, surveys are only good if they are actually returned. 

Hope this helps!  :)

The answer would be advertising.