Average Satisfactoin Rate

I need to conduct a customer satisifaction survey for a group a of departments. Each survey is similar in the criteria it is asking about (i.e. time/ quality/ employee behavior, etc) but the questions under each criteria are different based on the work nature of each department. In all surveys, there is a final question asking the respondent to rate his/her (total satisfaction rate) in egeneral.

1. How do I calculate the mean satisfaction rate from all surveys representing all department?

2. Do I have to make sure the number of questions under each criteria is similar in each department survey?

Thanks a lot for your help.