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Box and Whisker Plots

Steps to create a box plot for a set of data

1. Arrange the data in order from least to greatest
2. Find the median of the set. Data is now split into 2 sections.
3. Find the median of the lower half of the data and the median of the upper half of the data. Data is now split into 4 sections.
4. Identify the lower extreme (the least number) and the upper extreme (the greatest number.)
5. Above or below a number line, graph the 5 identified values.
6. Draw a box around the 3 medians and whiskers to the extremes.

Vocabulary

1st Quartile-the median of the lower half of the data

2nd Quartile-the median of the entire data set

3rd Quartile-the median of the upper half of the data

Lower Extreme-the number with the least value

Upper Extreme-the number with the greatest value

Range-the difference of the upper and lower extremes (the distance from the end of one whisker to the end of the other whisker

Interquartile Range-the difference of the 3rd and 1st quartiles (the width of the box)

Interpeting a box plot

Shows the distribution of the data. The data has been split into 4 equal parts. Therefore, each section represents one-fourth or 25% of the data.

Examples of possible interpetations:
75% of the data is larger than the 1st quartile's value
25% of the data is larger than the 3rd quartile's value
One-half of the data is smaller than the 2nd quartile's value
A whisker may be long due to an outlier
A quartile may be narrow due to the data values being close to each other
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