Glossary
Box Plot
A box plot is a graphical summary of a dataset's distribution. The box spans the interquartile range (IQR), with a line at the median. Whiskers typically extend to 1.5 times the IQR beyond the quartiles, and points beyond the whiskers are plotted individually as potential outl...
Definition
A box plot is a graphical summary of a dataset's distribution. The box spans the interquartile range (IQR), with a line at the median. Whiskers typically extend to 1.5 times the IQR beyond the quartiles, and points beyond the whiskers are plotted individually as potential outliers. Box plots enable quick visual comparison of location, spread, and symmetry across groups.
Why It Matters
Box plots provide a compact, standardised way to assess the centre, spread, and skewness of a distribution at a glance. They are especially valuable for comparing multiple groups side by side, revealing differences in median values, variability, and the presence of outliers. Because they rely on quartiles rather than means, box plots remain informative even when distributions are heavily skewed or contain extreme values.
Example
A researcher compares exam scores across five university departments. The box plots show that Department A has a higher median and tighter IQR (consistent performance), while Department C shows a lower median with a long upper whisker and several outliers, indicating wide variation in student performance. This visual comparison would be harder to discern from summary statistics alone.
Related Terms
Software Notes
- SPSS: Graphs > Chart Builder > Boxplot; drag variable to y-axis and grouping variable to x-axis
- R:
boxplot(value ~ group, data = df)for a grouped box plot - Stata:
graph box varname, over(groupvar)for side-by-side comparison