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Four levels, coded as 3, 1, 1, and 3 were chosen for each of two variables X1 and X2, to provide a total of sixteen experimental conditions when all possible combinations (X1,.X2} were taken. It was decided to use the resulting sixteen owervations to t a regression equation including a constant term, all possible rst-order, second-order, third-order and fourth-order terms in X1 and X2. The data were fed into a computer routine which ususlly obtains a vector estimate h = (x'xj-ix'Y The computer refused to obtain the estimates. \"Thy? The experimenter, who had meanwhile examined the data, decided at this stage to ignore the levels of variable X2 and t a fourth-order model in X1 onlyr to the same observations. The computer again refused to obtain estimates. my