Box plots and Outliers

Question 1

What is an outlier?

Answer 1

Any value that is more than 1.5 times the IQR below the lower quartile OR more than 1.5 times the IQR above the upper quartile

Question 2

What is the formula for finding:

1. A small outlier?
2. A large outlier?

Answer 2

1. Small outlier < (LQ - (1.5 x IQR)

2. Large outlier > (UQ + (1.5 x IQR)

Question 3

How can we use the standard deviation to find outliers?

Answer 3

An outlier is outside of : x̅ + OR - 3σ

where

• x̅ is the mean
• σ is the s.d.