List the values in the data set from lowest to highest value.
Divide the total number of values in the data set in half. Round up if the result is not a whole number. Use the result to know how many places to count in from the extremes. For example, if there are 25 values in the set, 25 / 2 equals 12.5. Round 12.5 up to 13 and count 13 places in from the left and the right.
Record the values you arrive at when counting in from the extremes. For example, if the data set is "1, 2, 3, 4," then you count in 2 values (since 4 values divided in half equals 2) to arrive at 2 from the left and 3 from the right.
Average the values you recorded to find the median. For example, 2 + 3 divided in half equals 2.5.
Repeat the above process to find the median values to the left and to the right of the median. For example, in "1, 2, 3, 4" the median is 2.5. Adding 1 plus 2 and dividing in half finds the median to the left of 2.5 (in this case 1.5). Repeat the process to the right of the median provides 3 plus 4 divided in half equals 3.5.
Record the median value found to the left of the median (1.5 in the example) as the lower quartile or Q1. Record the median value found to the right of the median (3.5 in the example) as the upper quartile or Q3.
Subtract the lower quartile from the upper quartile. For example, 3.5 minus 1.5 equals 2. Label the difference as the inner quartile range or IQR.