ANATOMY OF SYSTEMATIC RANDOM SAMPLING Systematic Random Sampling

ANATOMY OF SYSTEMATIC RANDOM SAMPLING Systematic Random Sampling provides an alternative to Simple Random Sampling. The determination of a sample using this method assures that members of the sample will be selected throughout the full range of the population, whereas Simple Random Sampling does not. Systematic Random Sampling provides results similar to Simple Random Sampling and may make obtaining a sample easier (quicker). Beware of using this sampling approach in situations of a cyclical nature. For example, a flaw in a machine may create a defect in every tenth product. If a Systematic RS is drawn starting with the eighth item and then every tenth, the defect would never occur in the sample and possibly lead to the conclusion that there were no defective items. Similarly if the tenth item was the starting point for the sample and followed by every tenth item, it would appear that all items were defective. The Process Identify the desired sample size and the population size. Divide the population size by the desired sample size and round DOWN to the nearest whole number, m. Use a random number table to obtain a number, k, between 1 and m. 40/10 = 4 m=4 (no rounding needed) For this example, the number randomly selected between 1 and 4 is 3=k Select the sample from the population whose members are: k, k+m, k+2 m, . . . The resulting sample is selected from throughout the range from 1 - 40 The Example We want to select a sample of 10, nickels from a roll (40 nickels) k = 3 is the first value selected; followed by k+m = 7, k +2 m = 11, etc. until 10 values are selected The Sample: 3, 7, 11, 15, 19, 23, 27, 31, 35, 39 The Sample Lay 40 nickels in a line and select those that meet the formula above. Sample Nickels: 3, 7, 11, 15, 19, 23, 27, 31, 35, 39. 1 2 3 4 | 5 6 7 8 | 9 10 11 12 | 13 14 15 16 | 17 18 19 20 | 21 22 23 24 | 25 26 27 28 | 29 30 31 32 | 33 34 35 36 | 37 38 39 40 k k+m k+2 m k+3 m k+4 m k+5 m k+6 m k+7 m k+8 m k+9 m