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Statistics

Drawing Box Plots

One way to make statistical data more accessible is to use charts. One type of chart that shows the median and the range of a data set is a box plot.

Concept

Five-Number Summary

A five-number summary contains five pieces of information, and is one way of describing the distribution of data in a data set.
  • minimum value
  • maximum value
  • median
  • first quartile
  • third quartile

Box plots are visual illustrations of this information.

Concept

Box Plot

A box plot can be used to illustrate the distribution of a data set. In the figure, the minimum, maximum, median, and first and third quartiles (Q1 and Q3) are shown.
box plot, between the values 1 to 10

A box plot provides a visual approximation of the distribution of a data set since each segment of the chart contains one quarter, or of the data. Note that the center of the data lies inside the box. The further apart the segments are, the greater the spread is for that quarter of the data.


fullscreen
Exercise

Thanks a Latte Café measured the temperature of 800 cappuccinos. The box plot below shows the results in F.

Box plot that shows the temperature of cappuccino

A good cappuccino, according to the experts, has a temperature between F and F. Approximately how many of the cappuccinos could be considered "good"?

Show Solution
Solution

Each part of a box plot contains of the data. This means the box contains of the data.

A box plot that shows the temperature of cappuccino
The first quartile is located at 129 degrees and the third is at 140 degrees. This means that about of the 800 cappuccionos were in the ideal temperature range. This gives
0.50800=400.
Approximately 400 cappuccinos can be considered "good".

Method

Drawing a Box Plot

Because a box plot shows the minimum, maximum, median, and first and third quartiles these need to be identified from the data set. The following data set gives the test scores for a grade.

1

Order the data set, then find the minimum and maximum
Sometimes, the data will be given in ascending order. When it is not, it's necessary to order the data so the quartiles can be found.
With an ordered data set, the minimum and maximum are easily identifiable. Here, the minimum is 5 and the maximum is 16. These are marked above a number line with a line segment, indicating the range.
The number line and the beginning of box plots showing maximum and minimum value


2

Determine the median
Since there are 26 values, the median is the mean of the numbers at the 13th and 14th position.
Now, the median can be determined by calculating the mean of 11 and 11.5.
The median is 11.25. This is marked as a vertical line segment in the range. Remember that the line for the median falls inside the box.
Started the box plot with minimum and maximum value and median


3

Determine the quartiles
Lastly, find the first and third quartiles. The median divides the set into two smaller sets each with 13 values. The first quartile is the middle value in the lower set: 8.5.
The third quartile is the median of the upper set: 13.5.


The quartiles are marked as the sides of the box plot. Now the box plot is complete.


Box plot between the values 5 and 16
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