The lower quartile ( Q 1or 25th percentile) is the median of the bottom half. The bottom half of this set consists of the first ten numbers (ordered from low to high): 280, 340, 440, 490, 520, 540, 560, 560, 580, and 580. The median of those ten is the average of the fifth and sixth scores—520 and 540—or 530. The lower‐quartile score is 530.
The upper quartile ( Q 3or 75th percentile) is the median score of the top half. The top half of this set consists of the last ten numbers: 600, 610, 630, 650, 660, 680, 710, 730, 740, and 740. The median of these ten is again the average of the fifth and sixth scores—in this case, 660 and 680—or 670. So 670 is the upper‐quartile score for this set of 20 numbers.
A box plot can now be constructed as follows: The left side of the box indicates the lower quartile; the right side of the box indicates the upper quartile; and the line inside the box indicates the median. A horizontal line is then drawn from the lowest value of the distribution through the box to the highest value of the distribution. (This horizontal line is the “whiskers.”)
Using the Verbal SAT scores in Table 1, a box plot would look like Figure 1.
Figure 1.A box plot of SAT scores displays median and quartiles.