The significance of sample size comes from manufacturing. This meant that to check for the accuracy of output per machine you need to audit x sample. i.e If Machine A produces 60,000 units you may want to audit 360 approx. sample. But this sample size is good for 60,000 units if produced by A. However if 60,000 are produced collectively by A & B then what should be the sample size.
The fact is if Machine A is 3 yrs old and Machine B is 6 years old. Should you -
Audit 360 overall
Base sample size basis individual production
Divide 360 proportionately basis production
The intent is to understand the impact of the variable on sample size. Also, do you recommend stratified sample rather than a random sample?