You just needed to say the value of the variables after each input. So, the mean variable stays 0 until -1 is entered. When -1 was entered, the mean is calculated, and it turned out to be 4 or 5 i think, can't quite remember.
For the data collection, I think I put interviews (both group/one-on-one) questionnaires, observation, and looking at documentation...
And for HCI, I put GUI for novices (well, all users), command for more experienced/expert users, Other input methods such as speed recognition to allow for the physically handicapped to use, and one other thing.