During the survey the interviewer must return to the Home Office at the end of each day if it is possible. The next day the questionnaires must be enter into the computer in order to check the validity of the fields. If something is wrong, the interviewer may still have fresh in memory the interview and provide the correct answer, or will return to the person to ask the proper questions again, only 2 days after.
Waiting for the end of the survey to correct mistakes generally proves impossible.