An interested student used the method of least squares to fit the straight line ŷ = 850. 7 + 320.3 x to gross domestic product, y, in real dollars. The results for 26 recent years, x = I, 2, ... , 26, appear here. Which assumption for a linear regression model appear to be seriously violated by the data?

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Year 1 NO 0 - 0 6 8 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 ŷ 1171.0 1491.4 1811.7 2132.0 2452.3 2772.6 3092.9 3413.3 3733.6 4053.9 4374.2 4694.5 5014.8 5335.2 5655.5 5975.8 6296.1 6616.4 6936.8 7257.1 7577.4 7897.7 8218.0 8538.3 8858.7 9179.0 Residual 466.7 333.2 218.4 161.8 109.9 15.5 33.9 -160.1 -199.0 -123.0 -156.7 -234.4 -278.4 -234.8 -173.4 -175.3 -304.0 -274.1 -269.4 -171.9 -162.7 -59.2 114.4 255.2 494.8 772.5