Suppose that 1,000 customers in an online electronics seller's customer database are categorized by three customer types: 200 potential customers who have visited the site but not purchased, 300 who have purchased once, and 200 repeat (loyal) buyers.
A sample of 100 customers is needed.

a. What type of sampling should you do? Why?

b. Explain how you would carry out the sampling according to the method stated in (a).

c. Why is the sampling in (a) not simple random sampling?