A binary operation is associative if rearranging the parentheses in an expression will not change the result
Example: + is associative; e.g., (2 + 3) + 4 = 2 + (3 + 4)
A binary operation is commutative if changing the order of the operands doesn't change the result
Example: + is commutative, / isn't commutative
x1: probability of p1 (e.g. 0.5)
Less sensitive to large outliers