The accompanying data resulted from a study of the relationship between brightness of finished paper (y) and the variables H2O2% by weight (x1), NaOH% by weight (x2), silicate % by weight (x3), and process temperature (x4)

(“Advantages of CEHDP Bleaching for High Brightness Kraft Pulp Production,” TAPPI, 1964: 170A–173A). Each independent variable was allowed to assume five different values, and these values were coded for regression analysis as 2, 1, 0, 1, and 2.