Add all of the test scores together. For example, 82 + 99 + 67 + 78 + 90 + 85 + 76 + 88 + 69 + 75 = 809.
Divide your result by the number of scores. For example, 809 ÷ 10 = 80.9. This is the mean score.
Subtract the mean score from each individual score. For example, 82 - 80.9 = 1.1; 99 - 80.9 = 18.1; 67 - 80.9 = -13.9; 78 - 80.9 = -2.9; 90 - 80.9 = 9.1; 85 - 80.9 = 4.1; 76 - 80.9 = -4.9; 88 - 80.9 = 7.1; 69 - 80.9 = -11.9; 75 - 80.9 = -5.9.
Square each of your result. For example, 1.1 * 1.1 = 1.21; 18.1 * 18.1 = 327.61; -13.9 * -13.9 = 193.21; -2.9 * -2.9 = 8.41; 9.1 * 9.1 = 82.81; 4.1 * 4.1 = 16.81; -4.9 * -4.9 = 24.01; 7.1 * 7.1 = 50.41; -11.9 * -11.9 = 141.61; -5.9 * -5.9 = 34.81.
Add your results. For example, 1.21 + 327.61 + 193.21 + 8.41 + 82.81 + 16.81 + 24.01 + 50.41 + 141.61 + 34.81 = 880.9.
Divide your result by the number of scores. For example, 880.9 ÷ 10 = 88.09.
Find the square root of your result. For example, √88.09 = 9.39. This is the standard deviation of your scores.
Divide each of your results from step 3 by the standard deviation. For example, 1.1 ÷ 9.39 = 0.12; 18.1 ÷ 9.39 = 1.93; -13.9 ÷ 9.39 = -1.48; -2.9 ÷ 9.39 = -0.31; 9.1 ÷ 9.39 = 0.97; 4.1 ÷ 9.39 = 0.44; -4.9 ÷ 9.39 = -0.52; 7.1 ÷ 9.39 = 0.76; -11.9 ÷ 9.39 = -1.27; -5.9 ÷ 9.39 = -0.63. These are the z-scores for each of the test scores.
Use each z-score to determine each test result's placement within the nine stanine categories. Z-scores of -1.75 or lower fall in group one; from -1.75 to -1.25 are group two; from -1.25 to -0.75 are group three; from -0.75 to -0.25 are group four; from -0.25 to 0.25 are group five; from 0.25 to 0.75 are group six; from 0.75 to 1.25 are group seven; from 1.25 to 1.75 are group eight and z-scores above 1.75 are in group nine.