I want to check the correctness of the data in a database with personal information like birthdate. So a survey is performed by interview and one of the questions was, "what is your date of birth". Comparing this answer with the information we have in our database will lead to an idea of the quality of the database. Out of a population of 1 mln people, lets say, 4.000 people are interviewed. Subtracting the two dates gives an idea of the error. Ideally is this zero for all the people in the survey. Which test can I use to say something about the significance of the error?