From time to time, a student will tell you that their data is incorrect. This may arise because ARCADE has emailed them, or through you asking them about irregularities when taking attendance. It is important to recognize that mistakes will be made, and they must be fixed as soon as possible. Generally, you ascertain whether the data really is wrong, and either explain to the student how they are misinterpreting it, or correct it. The printed sheets for all the previous laboratory sessions are available to you in the laboratory book, so that you can alter them. This is why you are asked not to use a black pen - red is best - so that such changes will not be missed when the team of data-entry demonstrators browses the old sheets.