Suppose that 5,000 sales invoices are separated into four strata. Stratum 1 contains 50 invoices, stratum 2 contains 500 invoices, stratum 1,000 invoices, and stratum 4 contains 3,450 invoices. A sample of 500 sales invoices is needed.
a) What type of sampling should you do? Why?
b) Explain how you would carry out the sampling according to the method in (a)