In most cases, a histogram analysis provides a sufficient display, but a box and whisker plot can provide additional detail while allowing multiple sets of data to be displayed in the same graph. Box and whisker plots are very effective and easy to read, as they can summarize data from multiple sources and display the results in a single graph. Box and whisker plots allow for comparison of data from different categories for easier, more effective decision-making. Use box and whisker plots when you have multiple data sets from independent sources that are related to each other in some way.
Examples include:. The procedure to develop a box and whisker plot comes from the five statistics below. You can also download the box and whisker plot template. Note: For a data set with an even number of values, the median is calculated as the average of the two middle values. Figure 1 Box and Whisker Plot Example. Lower quartile Twenty-five percent of scores fall below the lower quartile. Whiskers often but not always stretch over a wider range of scores than the middle quartile groups.
Box plots are used to show overall patterns of response for a group. They provide a useful way to visualise the range and other characteristics of responses for a large group. The diagram below shows a variety of different box plot shapes and positions.
Understanding and interpreting box plots. Definitions Median The median middle quartile marks the mid-point of the data and is shown by the line that divides the box into two parts. Some general observations about box plots The box plot is comparatively short — see example 2.
This suggests that overall students have a high level of agreement with each other. The box plot is comparatively tall — see examples 1 and 3.
This suggests students hold quite different opinions about this aspect or sub-aspect. One box plot is much higher or lower than another — compare 3 and 4 — This could suggest a difference between groups. Compare the interquartile ranges that is, the box lengths , to examine how the data is dispersed between each sample. The longer the box the more dispersed the data. The smaller the less dispersed the data. Next, look at the overall spread as shown by the extreme values at the end of two whiskers.
This shows the range of scores another type of dispersion. Larger ranges indicate wider distribution, that is, more scattered data.
If the data do not appear to be symmetric, does each sample show the same kind of asymmetry? McLeod, S. What does a box plot tell you? Toggle navigation. Statistics Box and Whisker Plot What does a box plot tell you? By Saul McLeod , published What is a box plot? Box plots are useful as they show the average score of a data set. Box plots are useful as they show the skewness of a data set.
The box plot shape will show if a statistical data set is normally distributed or skewed.
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