The Basic Practice of Statistics 7th Edition

The minimum and maximum values of the data set are 230 and 337, respectively. Since there are 20 values, the median value is the average of the 10th and 11th values, 319 and 320 (319.5). The $Q_{1}$ value is the median value between 230 and 319.5, and since there are eleven data points, the $Q_{1}$ value is the middle data point (the sixth data point), or 305. The $Q_{3}$ value is the median value between 319.5 and 337, and since there are eleven data points, the $Q_{3}$ value is the middle data point (the sixth data point), or 327.