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Math Help - Help with Time series

  1. #1
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    Exclamation Help with Time series

    Hi Guys,

    Could anyone help me with time series in stats?
    I have a homework to do but cant determine whether im supposed to use an additive model or a multiplicative one..

    Attached is a copy of the graph of data ive got to work with..
    Thank you very muchx

    Any help or suggestion will be welcomed and appreciated guys.

    Cheers,
    Amazed
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  2. #2
    Grand Panjandrum
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    Quote Originally Posted by Amazed View Post
    Hi Guys,

    Could anyone help me with time series in stats?
    I have a homework to do but cant determine whether im supposed to use an additive model or a multiplicative one..

    Attached is a copy of the graph of data ive got to work with..
    Thank you very muchx

    Any help or suggestion will be welcomed and appreciated guys.

    Cheers,
    Amazed
    It looks to me as though a linear trend with an additive seasonality signal will be a good model.

    RonL
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  3. #3
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    Hiya,

    Thanks very much Captain...but is there not an increase seasonality trend in the graph? Im am bit confused..

    Thanksxxx
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  4. #4
    Grand Panjandrum
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    Quote Originally Posted by Amazed View Post
    Hiya,

    Thanks very much Captain...but is there not an increase seasonality trend in the graph? Im am bit confused..

    Thanksxxx
    Not as far as I can see, but without the data I can't subtract off the trend to check, but the seasonality looks stationary to me.

    RonL
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  5. #5
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    hiya,

    If the datas available are in months, should i seperate them into quarters?(refer to the graph attached in original msg) I have to do them by hand and it seems to be a long procedure...
    i only have done example with quarterly months and dunno how to do with monthly data...

    I really appreciate ya help Captainblack and all others who viewed the thread.Ty

    Cheers,
    Amazed
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  6. #6
    Grand Panjandrum
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    Quote Originally Posted by Amazed View Post
    hiya,

    If the datas available are in months, should i seperate them into quarters?(refer to the graph attached in original msg) I have to do them by hand and it seems to be a long procedure...
    i only have done example with quarterly months and dunno how to do with monthly data...
    I would keep then in months, and use a period of 12 from the seasonality.

    RonL
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  7. #7
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    Quote Originally Posted by Amazed View Post
    Hi Guys,

    Could anyone help me with time series in stats?
    I have a homework to do but cant determine whether im supposed to use an additive model or a multiplicative one..

    Attached is a copy of the graph of data ive got to work with..
    Thank you very muchx

    Any help or suggestion will be welcomed and appreciated guys.

    Cheers,
    Amazed
    Attached is a spreadsheet showing the detrending of the underlying data, the extraction of the seasonality,
    and the reconstitution of the trend and seasonality, and the corresponding residuals. This is all assuming an
    additive model, where:

    sales = a*period + c + f(period)

    where f is a function of period which is perioodic with period 12 periods.

    The main plots are shown below:

    Note the residual plot shows a linear trend but with a low-ish value of R^2.

    RonL
    Attached Thumbnails Attached Thumbnails Help with Time series-gash1.jpg   Help with Time series-gash.jpg  
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  8. #8
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    Quote Originally Posted by CaptainBlack View Post
    Attached is a spreadsheet showing the detrending of the underlying data, the extraction of the seasonality,
    and the reconstitution of the trend and seasonality, and the corresponding residuals. This is all assuming an
    additive model, where:

    sales = a*period + c + f(period)

    where f is a function of period which is perioodic with period 12 periods.

    The main plots are shown below:

    Note the residual plot shows a linear trend but with a low-ish value of R^2.

    RonL
    Using the model in the previos post to perdict the next 12 months figures
    gives the prediction shown in the attachment:

    The first col (after the row number) is the period, the next is the trend, the
    third the seasonality, and the final col the trend plus the seasonality which is
    the prediction.

    From the residual plot we can see that we can expect the error in the predictions
    to be less than ~20, and usualy less than ~10.

    RonL
    Attached Thumbnails Attached Thumbnails Help with Time series-gash.jpg  
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