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Hi can anyone help me on this?

I am trying to understand how can I give a mathematical expression for the following Minitab output.

Sales=7.2 + 0.42Timesq + 27 Q1 + 56.2Q2 + 18.8Q3

I am confused with the fact that I have Time square. I know that if we have a quadratic trend we should use:

$y_{t=}\beta_0+\beta_1X_t+\beta_2X_t^2+\beta_3X_{s1,t+}\beta_4X_{s2,t}+\beta_5X_{s3,t}$

However, the Minitab output only shows timeSq. Should I use:

$y_{t=}\beta_0+\beta_1X_t^2+\beta_2X_{s1,t+}\beta_3X_{s2,t}+\beta_4X_{s3,t}$

insteade?

Thanks

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Use the equation as written, with just Timesq and not Time.

Either you did not ask for Time to be in the regression model OR you are using a stepwise procedure and Time was not selected.

If this is a forecasting application, the usual caveats about forecasting using a Time-squared variable should be taken seriously! You can easily produce strange forecasts.

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