Boxplots

boxplot-2

The Boxplot or Box and Whisker Plot is a graph that is used to show the range data and where the outliers are expected to arise.

The box section is the portion of the data between quartile 1 and quartile 3. There is generally a line inside the box, but not necessarily at the center, that indicates the second quartile.

  • The boxplot is used to depict trends in data and where data is expected to fall, but it can be misused if the statistician attempts to discern specific frequencies of occurrence from the boxplot itself.
  • The boxplot is useful for showing outliers and the range of the data. Outliers are graphed as dots outside the whiskers (lines).
  • The boxplot is useful for showing the median in a data set.

The most common occurrence does not necessarily fall at the median, where the line is drawn in the box. A quartile is a section of the data where we expect to find a quarter of a random sampling of data.

Outside the lines are where outliers are graphed, and they are generally plotted along with the boxplot in the form of a dot. Alternative forms have the lines including ranges inside a standard deviation or inside a certain specified range. In a box and whisker plot where there are no outliers, the first quarter of data, between the 1st and 25th percentile, is the section from the line to the start of the box. The box then consists of 25th-75th percentile, and the rightmost (or top or bottom, depending on the format) line is from the 75th-100th percentile

Here is a useful site with a short video description of boxplots. Here is a site that allows the user to manipulate data and become comfortable with the formatting.

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One comment

  1. […] side-by-side boxplot has all the advantages of a single boxplot (which can be seen here) with the added benefit of providing clear comparisons between levels […]

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