- Box plots and their parameters are generally used if data is not known to be normal and they assist in displaying how the
data is distributed.
- The most basic parameters for a box-plot are:
- xlow, xmin, lowest score,
- 1st quartile (Q1, 25th percentile, median of the lower 50% of all data),
- 2nd quartile (Q2, median, 50th percentile),
- 3rd quartile (Q3, 75th percentile, median of the upper 50% of all data),
- xhigh, xmax, highest score,
- inner quartile range, IQR, Q3 - Q1.
- Box plots are usually drawn to scale so the relationship between the data points is visible. Draw the number line first, then,
create the box plot.
- Here's an example.
- As with any boxplot, the middle 50% of all the data points fall in the inner quartile range, Q3 - Q1. In
this case the IQR is 8-3, or 5.