What is a box plot?
The type of graph used above is called a box plot. A box plot, or a box-and-whisker diagram, is used to compare distributions of observed values. It is a useful way to show groups of numerical data through “five-number summaries.” These five values make up the five-number summaries: • Minimum: the smallest observed value • Lower quartile or 25th percentile: the value below which one quarter of all observed values occur • Median: the central value below which half the observed values occur • Upper quartile or 75th percentile: the value below which three quarters of all observed values occur • Maximum: the largest observed value What does a box plot show? The box is the area between the 25th and 75th percentile values. The whisker is the perpendicular line extending through the box. Observed values that fall along the length of the whisker are not considered significantly different from the median. Random chance can explain relatively small differences between those values. Values beyond
