Appraisals for field staff - don't want to give equal appraisals to each member

I am working in a not for profit company in which we have 20 members field team and for this year we have to do appraisals of team. But i am too confused that on what basis i have to start work for appraisals?. Basically we are not working on any kind of target oriented work also don't having specific work profile for everyone, every member is capable to work in each section and doing the same work. We don't want to give equal appraisals to each member.
Only attendance, behaviour are not too much sufficient criteria to do different categories of appraisals. So please give me your opinion for deciding the process of appraisals.

Hello Hratul,
Considering the facts that you have mentioned above, Appraisals are essential to know the performers and non-performers. There must be existing Appraisal system in the organization or are you initiating it? I think it is imperative to understand the motive of your employers behind the Appraisals idea.
Even if you are doing the same work, who is 'accountable' to what should be certainly known to your field staff as well as employers.
I am an HR Consultant from Pune. If you are open to services from a Consultant for Appraisal work, please contact me.