usually for your convenience, you may apply 5 parameters, in which 3 will be general in application and two in specific, it is not possible to give you in exact form as we do not know what kind of work and projects the employees are on....... how many are completed and about to complete, to take in to consideration, and is it just for the present year or does it include the past performances like the previous yea, special projects etc......
Very simple, you will surely know the general parameters with which you shall identify the very important few which specifically goes with such projects/jobs, may be for example like team co ordination, knowledge orientation, time period of completion and quality...... if you may refer the assessment check lists, are available in plenty online and also on Cite, it wont be a big problem at all.......
It seems your mind is preoccupied with the concepts called difficult, confusing, mixture of projects, kind of employees.......I suggest you stop completely thinking about it today, put a lock to your that part of brain, by tomorrow it will be subtle and you will automatically complete it asap..............