Quartile Deviation and its Coefficient

Quartile Deviation (Q.D.) is a measure of dispersion that tells us how much the middle 50% of the data is spread out.

It is based on two quartiles:

• First Quartile (Q₁) → 25% of the observations lie below it.
• Third Quartile (Q₃) → 75% of the observations lie below it.

So, the data between Q₁ and Q₃ represents the middle 50% of the observations.

Formula for Quartile Deviation

Formula for Coefficient of Quartile Deviation