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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 or box and whisker plot can be used to illustrate the distribution of a data set. A box plot has three parts.

  • A rectangular box that extends from the first to the third quartiles and with a line in the middle indicating the position of the median.
  • A segment attached to the left of the box that extends from the first quartile to the minimum of the data set.
  • A segment attached to the right of the box that extends from the third quartile to the maximum of the data set.

A boxplot is a scaled figure, usually presented above a number line.

Boxplot shown above a number line.
A box plot provides a visual illustration of the five-number summary of a data set. Each segment of the chart contains one quarter, or of the data, and 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 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 degrees and the third is at degrees. This means that about of the cappuccionos were in the ideal temperature range. This gives Approximately 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 and the maximum is 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 values, the median is the mean of the numbers at the th and th position. Now, the median can be determined by calculating the mean of and The median is 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 values. The first quartile is the middle value in the lower set: The third quartile is the median of the upper set:


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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