Bob is selecting a sample of sales invoices for substantive testing. All documents relating to sales invoices for the last five years are stored in boxes in the shed behind the office. The shed is very small and the boxes are stacked on top of each other because the shelves are full. Due to the damp conditions, some labels have peeled from the boxes, so it is not clear which boxes relate to the current year. 

Required: 

(a) Describe the population(s) that would be relevant to Bob's sample.

b) Which sample selection methods would be appropriate for choosing sales invoices for substantive testing at RGHF? Explain the factors that would influence your choice.