Mistake in ASET (S'03 Q13)

Discussion in 'CT6' started by Rosencruz, Aug 31, 2006.

  1. Rosencruz

    Rosencruz Member

    Looks like this is wrong to me. Firstly the answer says that you need to take both seasonal differences andnormal differences. However, surely seasonal differencing will remove the linear trend as well?
    Secondly in the later part where the models are fitted they get the SACF an dth SPACF round the wrong way, and fit the MA part using the SPACF, and the AR part with the SACF?
    Surely this bit of the ASET must have been written late at night?
     
    Rosencruz, Aug 31, 2006
    #1
  2. Rosencruz

    Rosencruz Member

    Sorry thats solution to question 12
     
    Rosencruz, Aug 31, 2006
    #2
  3. John Lee

    John Lee ActEd Tutor Staff Member

    Yup part (iii)(b) the MA and AR are the wrong way round. Apologies.

    Seasonal differencing doesn't necessarily remove the linear trend. Seasonal differencing on its own in this case will not produce a stationary time series. However, differencing then seasonal differencing will. Easier to show on a graph - email me at johnlee*bpp.com if you want to see it
     
    Last edited: Sep 25, 2006
    John Lee, Sep 25, 2006
    #3

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