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David, In my opinion this is a very bad model. There is no justification whatsoever for a seasonal differencing of order 6 for the time series that you analyzed using SPSS. Furthermore there are more egregious outliers/interventions than the one at time period "10". Hope this helps. Dave Reilly Senior Vice President Automatic Forecasting Systems www.autobox.com 215-675-0652 (office) 215-353-7087 (cell) -----Original Message----- From: A UK-based worldwide e-mail broadcast system mailing list [mailto:[log in to unmask]] On Behalf Of [log in to unmask] Sent: Thursday, December 10, 2009 10:11 AM To: [log in to unmask] Subject: SPSS Time Series Output I hope someone can help or direct me somewhere useful I have a table from SPSS Time Series Analysis -version 16. I can confirm the periodicity of the data is 12 as it is monthly. I have an ARIMA model with Seasonal Differencing. It says 1 under the estimate heading i.e. one degree of seasonal differencing. But what does a figure of 6 mean earlier in the row? There is another row with similar information at the end which refers to a significant intervention variable. Again, what does a 10 mean here? Excuse ignorance Dr David Pitfield AcSS Senior Lecturer Transport Studies Group Room RT008 Department of Civil and Building Engineering Loughborough University Loughborough Leicestershire LE11 3TU UK Phone +44 1509 223416 Fax +44 1509 223981 Email [log in to unmask] URL : http://www.lboro.ac.uk/departments/cv/staff/profile/38.html Home: Lings Farm 47 Main Street Keyworth Nottingham NG12 5AA UK Phone +44 115 9374922 Cell +44 7765 783300 Email [log in to unmask]