Summation in all cases is over i= 1 to n, the n observations in the data. When only one independent variable is involved, X'X consists of only the upper-left 2 × 2 matrix. Inspection of the normal equations in Chapter 1, equation 1.6, reveals that the elements in this 2 × 2 matrix are the coefficients on β ̂_0and β ̂_1.