15. You are given a set of m objects that is divided into k groups, where the ith group is of size m_i . If the goal is to obtain a sample of size n < m , what is the difference between the following two sampling schemes? (Assume sampling with replacement.)

a) We randomly select n m i / m elements from each group.

b) We randomly select n elements from the data set, without regard for the group to which an object belongs.