Construct and interpret a box plot

The box extends from Q1 to Q3, with a line at the median. IQR = Q3-Q1. Inner fences are Q1-1.5 IQR and Q3+1.5 IQR. In the modified box plot used here, whiskers end at the most extreme observed values still inside these fences; points beyond are shown individually. Outer fences at 3 IQR may identify especially extreme points. An outlier is a signal for investigation, not automatic evidence of error.