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7.11: Box-and-Whisker Plots

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You're a meteorologist and you're collecting temperature data from various locations in your state for the month of February. You've collected over 2,000 temperatures at the same time each day and want to organize them to see if there are patterns. You want to find out the lowest temperature, the highest temperature, the median temperature, the median of the first half of the month and the median of the second half of the month. How would you organize your data to get these answers?

Watch This

First watch this video to learn about box-and-whisker plots.

CK-12 Foundation: Chapter7BoxandWhiskerPlotsA

Then watch this video to see some examples.

CK-12 Foundation: Chapter7BoxandWhiskerPlotsB

Watch this video for more help.

Khan Academy Box-and-whisker Plot

Guidance

In traditional statistics, data is organized by using a frequency distribution. The results of the frequency distribution can then be used to create various graphs, such as a histogram or a frequency polygon, which indicate the shape or nature of the distribution. The shape of the distribution will allow you to confirm various conjectures about the nature of the data.

To examine data in order to identify patterns, trends, or relationships, exploratory data analysis is used. In exploratory data analysis, organized data is displayed in order to make decisions or suggestions regarding further actions. A box-and-whisker plot (often called a box plot) can be used to graphically represent the data set, and the graph involves plotting 5 specific values. The 5 specific values are often referred to as a five-number summary of the organized data set. The five-number summary consists of the following:

  1. The lowest number in the data set (minimum value)
  2. The median of the lower quartile: Q_1 (median of the first half of the data set)
  3. The median of the entire data set (median)
  4. The median of the upper quartile: Q_3 (median of the second half of the data set)
  5. The highest number in the data set (maximum value)

The display of the five-number summary produces a box-and-whisker plot as shown below:

The above model of a box-and-whisker plot shows 2 horizontal lines (the whiskers) that each contain 25% of the data and are of the same length. In addition, it shows that the median of the data set is in the middle of the box, which contains 50% of the data. The lengths of the whiskers and the location of the median with respect to the center of the box are used to describe the distribution of the data. It's important to note that this is just an example. Not all box-and-whisker plots have the median in the middle of the box and whiskers of the same size.

Information about the data set that can be determined from the box-and-whisker plot with respect to the location of the median includes the following:

a. If the median is located in the center or near the center of the box, the distribution is approximately symmetric.

b. If the median is located to the left of the center of the box, the distribution is positively skewed.

c. If the median is located to the right of the center of the box, the distribution is negatively skewed.

Information about the data set that can be determined from the box-and-whisker plot with respect to the length of the whiskers includes the following:

a. If the whiskers are the same or almost the same length, the distribution is approximately symmetric.

b. If the right whisker is longer than the left whisker, the distribution is positively skewed.

c. If the left whisker is longer than the right whisker, the distribution is negatively skewed.

The length of the whiskers also gives you information about how spread out the data is.

A box-and-whisker plot is often used when the number of data values is large. The center of the distribution, the nature of the distribution, and the range of the data are very obvious from the graph. The five-number summary divides the data into quarters by use of the medians of the upper and lower halves of the data. Many data sets contain values that are either extremely high values or extremely low values compared to the rest of the data values. These values are called outliers . There are several reasons why a data set may contain an outlier. Some of these are listed below:

  1. The value may be the result of an error made in measurement or in observation. The researcher may have measured the variable incorrectly.
  2. The value may simply be an error made by the researcher in recording the value. The value may have been written or typed incorrectly.
  3. The value could be a result obtained from a subject not within the defined population. A researcher recording marks from a math 12 examination may have recorded a mark by a student in grade 11 who was taking math 12.
  4. The value could be one that is legitimate but is extreme compared to the other values in the data set. (This rarely occurs, but it is a possibility.)

If an outlier is present because of an error in measurement, observation, or recording, then either the error should be corrected, or the outlier should be omitted from the data set. If the outlier is a legitimate value, then the statistician must make a decision as to whether or not to include it in the set of data values. There is no rule that tells you what to do with an outlier in this case.

One method for checking a data set for the presence of an outlier is to follow the procedure below:

  1. Organize the given data set and determine the values of Q_1 and Q_3 .
  2. Calculate the difference between Q_1 and Q_3 . This difference is called the interquartile range (IQR) : IQR = Q_3-Q_1 .
  3. Multiply the difference by 1.5, subtract this result from Q_1 , and add it to Q_3 .
  4. The results from Step 3 will be the range into which all values of the data set should fit. Any values that are below or above this range are considered outliers.

Example A

For each box-and-whisker plot, list the five-number summary and describe the distribution based on the location of the median.

a. Minimum value \rightarrow 4

Q_1 \rightarrow 6

Median \rightarrow 9

Q_3 \rightarrow 10

Maximum value \rightarrow 12

The median of the data set is located to the right of the center of the box, which indicates that the distribution is negatively skewed.

b. Minimum value \rightarrow 225

Q_1 \rightarrow 250

Median \rightarrow 300

Q_3 \rightarrow 325

Maximum value \rightarrow 350

The median of the data set is located to the right of the center of the box, which indicates that the distribution is negatively skewed.

c. Minimum value \rightarrow 60

Q_1 \rightarrow 70

Median \rightarrow 75

Q_3 \rightarrow 95

Maximum value \rightarrow 100

The median of the data set is located to the left of the center of the box, which indicates that the distribution is positively skewed.

Example B

The numbers of square feet (in 100s) of 10 of the largest museums in the world are shown below:

650, 547, 204, 213, 343, 288, 222, 250, 287, 269

Construct a box-and-whisker plot for the above data set and describe the distribution.

The first step is to organize the data values as follows:

20,400 \quad 21,300 \quad 22,200 \quad 25,000 \quad 26,900 \quad 28,700 \quad 28,800 \quad 34,300 \quad 54,700 \quad 65,000

Now calculate the median, Q_1 , and Q_3 .

20,400 \quad 21,300 \quad 22,200 \quad 25,000 \quad \boxed{26,900 \quad 28,700} \quad 28,800 \quad 34,300 \quad 54,700 \quad 65,000

\text{Median} \rightarrow \frac{26,900+28,700}{2} = \frac{55,600}{2} = 27, 800

Q_1 = 22,200

Q_3 = 34,300

Next, complete the following list:

Minimum value \rightarrow 20,400

Q_1 \rightarrow 22,200

Median \rightarrow 27,800

Q_3 \rightarrow 34,300

Maximum value \rightarrow 65,000

The right whisker is longer than the left whisker, which indicates that the distribution is positively skewed.

Example C

Using the procedure outlined above, check the following data sets for outliers:

a. 18, 20, 24, 21, 5, 23, 19, 22

b. 13, 15, 19, 14, 26, 17, 12, 42, 18

a. Organize the given data set as follows:

& 18, \ 20, \ 24, \ 21, \ 5, \ 23, \ 19, \ 22\\& 5, \ 18, \ 19, \ 20, \ 21, \ 22, \ 23, \ 24

Determine the values for Q_1 and Q_3 .

5, \ \boxed{18, \ 19}, \ 20, \ 21, \ \boxed{22, \ 23}, \ 24

Q_1 = \frac{18+19}{2} = \frac{37}{2}= 18.5 \qquad Q_3=\frac{22+23}{2}=\frac{45}{2}=22.5

Calculate the difference between Q_1 and Q_3 : Q_3-Q_1=22.5-18.5=4.0 .

Multiply this difference by 1.5: (4.0)(1.5)=6.0 .

Finally, compute the range.

Q_1-6.0=18.5-6.0=12.5

Q_3+6.0=22.5+6.0=28.5 .

Are there any data values below 12.5? Yes, the value of 5 is below 12.5 and is, therefore, an outlier.

Are there any values above 28.5? No, there are no values above 28.5.

b. Organize the given data set as follows:

& 13, \ 15, \ 19, \ 14, \ 26, \ 17, \ 12, \ 42, \ 18\\& 12, \ 13, \ 14, \ 15, \ 17, \ 18, \ 19, \ 26, \ 42

Determine the values for Q_1 and Q_3 .

12, \ \boxed{13, \ 14}, \ 15, \ \boxed{17}, \ 18, \ \boxed{19, \ 26}, \ 42

Q_1=\frac{13+14}{2} = \frac{27}{2}=13.5 \qquad Q_3 = \frac{19+26}{2} = \frac{45}{2} = 22.5

Calculate the difference between Q_1 and Q_3 : Q_3-Q_1=22.5-13.5=9.0 .

Multiply this difference by 1.5: (9.0)(1.5)=13.5 .

Finally, compute the range.

Q_1-13.5=13.5-13.5=0

Q_3+13.5=22.5+13.5=36.0

Are there any data values below 0? No, there are no values below 0.

Are there any values above 36.0? Yes, the value of 42 is above 36.0 and is, therefore, an outlier.

Points to Consider

  • Are there still other ways to represent data graphically?
  • Are there other uses for a box-and-whisker plot?
  • Can box-and-whisker plots be used for comparing data sets?

Guided Practice

a. For the following data sets, determine the five-number summaries:

i. 12, 16, 36, 10, 31, 23, 58

ii. 144, 240, 153, 629, 540, 300

b. Use the data set for part i of the previous question and the five-number summary to construct a box-and-whisker plot to model the data set.

Answer:

a. i. The first step is to organize the values in the data set as shown below:

&12, \ 16, \ 36, \ 10, \ 31, \ 23, \ 58\\&10, \ 12, \ 16, \ 23, \ 31, \ 36, \ 58

Now complete the following list:

Minimum value \rightarrow 10

Q_1 \rightarrow 12

Median \rightarrow 23

Q_3 \rightarrow 36

Maximum value \rightarrow 58

ii. The first step is to organize the values in the data set as shown below:

&144, \ 240, \ 153, \ 629, \ 540, \ 300\\&144, \ 153, \ 240, \ 300, \ 540, \ 629

Now complete the following list:

Minimum value \rightarrow 144

Q_1 \rightarrow 153

Median \rightarrow 270

Q_3 \rightarrow 540

Maximum value \rightarrow 629

b. The five-number summary can now be used to construct a box-and-whisker plot for part i. Be sure to provide a scale on the number line that includes the range from the minimum value to the maximum value.

Minimum value \rightarrow 10

Q_1 \rightarrow 12

Median \rightarrow 23

Q_3 \rightarrow 36

Maximum value \rightarrow 58

It is very visible that the right whisker is much longer than the left whisker. This indicates that the distribution is positively skewed.

Interactive Practice

Practice

  1. Which of the following is not a part of the five-number summary?
    1. Q_1 and Q_3
    2. the mean
    3. the median
    4. minimum and maximum values
  2. What percent of the data is contained in the box of a box-and-whisker plot?
    1. 25%
    2. 100%
    3. 50%
    4. 75%
  3. What name is given to the horizontal lines to the left and right of the box of a box-and-whisker plot?
    1. axis
    2. whisker
    3. range
    4. plane
  4. What term describes the distribution of a data set if the median of the data set is located to the left of the center of the box in a box-and-whisker plot?
    1. positively skewed
    2. negatively skewed
    3. approximately symmetric
    4. not skewed
  5. What 2 values of the five-number summary are connected with 2 horizontal lines on a box-and-whisker plot?
    1. Minimum value and the median
    2. Maximum value and the median
    3. Minimum and maximum values
    4. Q_1 and Q_3
  6. For the following data sets, determine the five-number summaries:
    1. 74, 69, 83, 79, 60, 75, 67, 71
    2. 6, 9, 3, 12, 11, 9, 15, 5, 7
  7. For each of the following box-and-whisker plots, list the five-number summary and comment on the distribution of the data:
  8. The following data represents the number of coins that 12 randomly selected people had in their piggy banks: 35 \quad 58 \quad 29 \quad 44 \quad 104 \quad 39 \quad 72 \quad 34 \quad 50 \quad 41 \quad 64 \quad 54 Construct a box-and-whisker plot for the above data.
  9. The following data represent the time (in minutes) that each of 20 people waited in line at a local book store to purchase the latest Harry Potter book: & 15 \quad 8 \quad 5 \quad \ 10 \quad 14 \quad 17 \quad 21 \quad 23 \quad 6 \quad 19 \quad 31 \quad 34 \quad 30 \quad 31\\& 3 \quad 22 \quad 17 \quad 25 \quad 5 \quad 16 Construct a box-and-whisker plot for the above data. Are the data skewed in any direction?
  10. Firman’s Fitness Factory is a new gym that offers reasonably-priced family packages. The following table represents the number of family packages sold during the opening month: & 24 \quad 21 \quad 31 \quad 28 \quad 29\\& 27 \quad 22 \quad 27 \quad 30 \quad 32\\& 26 \quad 35 \quad 24 \quad 22 \quad 34\\& 30 \quad 28 \quad 24 \quad 32 \quad 27\\& 32 \quad 28 \quad 27 \quad 32 \quad 23\\& 20 \quad 32 \quad 28 \quad 32 \quad 34 Construct a box-and-whisker plot for the data. Are the data symmetric or skewed?
  11. Shown below is the number of new stage shows that appeared in Las Vegas for each of the past several years. Construct a box-and-whisker plot for the data and comment of the shape of the distribution. 31 \quad 29 \quad 34 \quad 30 \quad 38 \quad  40 \quad 36 \quad 38 \quad 32 \quad 39 \quad 35
  12. The following data represent the average snowfall (in centimeters) for 18 Canadian cities for the month of January. Construct a box-and-whisker plot to model the data. Is the data skewed? Justify your answer.
Name of City Amount of Snow(cm)
Calgary 123.4
Charlottetown 74.5
Edmonton 80.6
Fredericton 73.8
Halifax 64.0
Labrador City 110.4
Moncton 82.4
Montreal 63.6
Ottawa 48.9
Quebec City 53.8
Regina 35.9
Saskatoon 25.4
St. John’s 97.5
Sydney 44.2
Toronto 21.8
Vancouver 12.8
Victoria 8.3
Winnipeg 76.2
  1. Using the procedure outlined in this concept, check the following data sets for outliers:
    1. 25, 33, 55, 32, 17, 19, 15, 18, 21
    2. 149, 123, 126, 122, 129, 120

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