Operational Governance Knowledge Flashcards No. 04/19
Box Plot Charts
From the Operational Governance flashcard deck; today this discipline goes by BizOps. The original card is republished here as a field note.
01 What is it
Boxplots are a standardized way of displaying the distribution of data based on a five number summary: “minimum”, first quartile (Q1), median, third quartile (Q3), and “maximum”.
They give a good graphical image of the concentration of the data and also show how far the extreme values are from most of it.
02 When is it useful
When indicating whether a distribution is skewed and whether there are potential unusual observations (outliers) in the data set.
When large numbers of observations are involved, and when two or more data sets are being compared — the centre, spread, and overall range are immediately apparent.
03 How to use it
- Using the equal interval scale, draw a rectangular box with one end at Q1 and the other end at Q3.
- Draw a vertical segment at the median value.
- Draw two horizontal segments on each side of the box, one down to the minimum value and one up to the maximum value. These are the “whiskers”.
04 The anatomy of a box plot
05 Common pitfalls
- Missing data when ordering the data; using the mean instead of the median; and similar slips.
- Summarizing means losing information: you can no longer see the underlying distribution of dots in each group, or their number of observations.