Presume that 5,000 sales invoices are separated into four strata. Stratum 1 contains 50 invoices stratum 2 contains 500 invoices stratum 3 contains 1,000 invoices and stratum 4 contains 3,450 invoices. A example of 500 sales invoices is needed.
a. What kind of sampling should you do? Why?
b. Describe how you would carry out the sampling according to the method in (a)
c. Why is the sampling is not simple random sampling?