a) Divide the population units into two strata with the first stratum containing the front ( ) n rk −units and second stratum housing the remaining ( ) 1 rk+ units. From stratum I, a random start
k is selected from the interval 1 to k and every k units thereafter, from the ( ) nr− group of units forming stratum I. Thus samples from stratum I contained in a sample space S′ are: