yi =β0+β 1x 1i+β2X2i+…+βkXki+μI I = 1，2，…，n

Where k is the number of explanatory variables, β j (j = 1, 2, ..., k) is called the regression coefficient. The above formula is also called the random expression of the population regression function. Its non-random expression is

e(y∣x 1i,x2i,…xki,)=β0+β 1x 1i+β2x2i+…+βkxki

βj is also called partial regression coefficient.