As written your question is far too vague for anyone to be able to offer much in the way of help.
Questions:
1. What kind of a population? Bunch of guys and gals stuffed into a single room, multiple centers within a company, multiple centers across a number of corporate centers in different geographical locations, etc.?
2. What kind of calls? All the same type, different methods/styles for various products, projects, groups, etc.
3 Are the calls solicitation calls or are they in response to customer questions?
4. If solicitations – what time for the calls – 24/7 – between the hours of 12 midnight and 2AM, etc.
5. If response to questions – what kinds of categories of questions?
6. If response to questions – how are you classifying question difficulty?
…and so on – in short, we need a description of the process and a statement concerning the aspect of the process you are most interested in evaluating.
Sample size – again without knowing what it is that you want to do the simple, but accurate, statement about sample size is one sample per person. What are you looking for, what are you measuring, have you done your homework with regard to the connection between what it is that you plan to measure and what it is that you plan to do with the measurements once you have them,….etc.
If you can provide more details perhaps I or someone else might be able to offer some suggestions/answers.
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