text: Zero.  I'm being a jerk, but for an important reason.  Your question is incomplete.  You can ask two people and have a 95% confidence that the results are representative.  But your margin of error is 75 percent.  That's the part I'm being a jerk about.\n\nIf you're trying to predict an election, 3 or 5% is probably close enough.  But that means a 55 to 45 vote is as likely as a 45 to 55 vote.    And 3% means asking more than 800 people, 5% only 350.  If you sell extended computer warranties and need to know how often their pc breaks, a smaller error is very important.\n\nYou can use an online tool to calculate these figures [click below] or you can skip the Audi TT and go to graduate school to learn how to do it yourself [damn! there was an online tool?].  But this is only an estimate.  The actual figures depend on the question and consistency of the answers.