The random vector ε is a vector( ε_1 , ε_2 … ε_n )^'consisting of random variables ε_i.
Since the elements of X and β are assumed to be constants, the Xβ term in the model is a vector of constants. Thus, Y is a random vector that is the sum of the constant vector Xβ and the random vector ε. Since ε_iare assumed to be independent N(0,σ^2) random variables, we have that