Calculate the average score, or mean, by adding all the scores and then dividing the sum by the number of scores. Given scores of 23, 32, 37, 39, 39, 42, 45 and 49 on a test with a maximum of 50 points, add 23, 32, 37, 39, 39, 42, 45 and 49 to get 307. Divide by 307 by 8 to get a mean of 38.37.
Determine the middle score, or median, by selecting the middle score. If there is no middle score, then find the mean of the two most middle scores. Given the example, the middle scores are 39 and 39. Since they are both the same, 39 is the median.
Derive the spread, or standard, deviation by squaring the mean, squaring each individual score, adding the squared scores, dividing the sum of squared scores by the number of scores, subtracting this result from the squared mean and taking the square root of the previous result's absolute value. Given the example, square the mean 38.37 to get 1472.26, square the scores and add them together to get 12,154.00, divide the sum by 8 to get 1519.25, subtract the sum 1519.25 from the mean 1472.26 to get -46.99. Take the square root of the absolute value of -46.99 to get a standard deviation of 6.85.
Add the rounded value of the standard deviation to the rounded value of the median to arrive at the 90 percent grading value. Given the example, add 7 (6.85 rounded up) to the median 39 to get 46. This means that any student who scored 46 or more points earned an A.
Subtract the rounded value of the standard deviation from the rounded value of the median to arrive at the 70 percent grading value. Given the example, subtract 7 from 39 to get 32. This means that any student who scored between 32 and 39 points earned a C, while any students who scored between 39 points and 46 points earned a B.
Keep subtracting the median by multiples of the standard deviation to arrive at the 60 percent and 50 percent grading values. Given the example, subtract the standard deviation's double (7 times 2) from the median 39 to arrive at the 60 percent grading value. Likewise, subtract its triple (7 times 3) from the median to arrive at the 50 percent grading value.