To compute the third quartile:

1. Sort your data in increasing order.

2. Determine the upper half of your data:

   • Even number of observations: Split your data set in half and take the upper half.

   • Odd number of observations: Take all the numbers bigger than the number in the middle of your set.

3. Compute the upper quartile as the median of the upper half:

   • Odd number of observations: Take the number right in the middle.

   • Even number of observations: Take the average of the two values in the middle.

The third quartile is the right side of the box (if horizontal) or the top side (if vertical). The left/bottom side corresponds to the first quartile. Hence, the box's width is exactly the interquartile range (IQR).

Q3 = 0.67448 in the standard normal distribution. In any normal distribution, Q3 = μ + 0.67448σ, where μ is the mean and σ is the standard deviation of the normal distribution in question.