What is the outlier in 2 3 18 18 18 19 20 20 21 21?

Answer 1

There are no universally agreed determinants are outliers.

Commonly used measure are

Lower Outer Fence : Q1 - 3*IQR

Lower Inner Fence : Q1 - 1.5*IQR

Upper Inner Fence : Q3 + 1.5*IQR

Upper Outer Fence : Q3 + 3*IQR

where Q1 and Q3 are the lower and upper quartiles and IQR = Q3 - Q1.

Values further than the outer fence are called extreme outliers while those between the inner and outer fences are mild outliers.

On that basis the values 2 and 3 are both mild outliers.