Mean Deviation (MD) = sum$\frac{|xi - A|}{n}$, where A is typically the mean or median. MD about median is always less than or equal to MD about mean — this is a key theoretical fact. MD uses absolute values, making it harder to manipulate algebraically compared to variance (which uses squared deviations). For this reason, variance is preferred in advanced statistics. JEE tests MD rarely but may include it as an option in "which statement is true" type MCQs. MD gives equal weight to all deviations regardless of sign, unlike variance which gives more weight to larger deviations.