An analyst studies quarterly data on the relationship between retail sales (y, in $ millions), gross national product (x, in $ billions), and a quarterly dummy d that equals 1 if the sales are for the 4th quarter, 0 otherwise. He estimates the model y = ?₀ + ?₁x + ?₂d + ?₃xd + ?. Relevant regression results are shown in the accompanying table.

a. Interpret the dummy variable, d. Is it significant at the 5% level?

b. Interpret the interaction variable. Is it significant at the 5% level?

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p-Value Coefficients Standard Error 186553.3 t Stat 0.002 3.31 Intercept 56421.1 12.08 55.0 4.6 х 0.000 0.342 112605.8 117053.0 0.96 -4.7 9.3 0.618 -0.50 xd