The type of sampling method used is Stratified Sampling: Stratified Sampling is where the population has a number of distinct categories (different age groups in Billy's case). Here, the sampling frame can be organised into separate categories( age groups) which is called 'strata'. Each stratum is then sampled as an independent sub-population out of which individual elements can be randomly selected. In this case, each element in a group has a chance of being selected. With Stratified sampling, the best survey result occurs when elements within the strata are internally homogeneous ( in this case, the customers from each age group are selected).
