 In straight line fitting, the model is
where formula_2 is the response variable, formula_3 is the explanatory variable, "εi" is the random error, and formula_4 and formula_5 are parameters.The mean, and predicted, response value for a given explanatory value, "xd", is given by

while the actual response would be

Expressions for the values and variances of formula_8 and formula_9 are given in linear regression.Since the data in this context is defined to be ("x", "y") pairs for every observation, the "mean response" at a given value of "x", say "xd", is an estimate of the mean of the "y" values in the population at the "x" value of "xd", that is formula_10.