Figure P-23 is a time series plot of the weekly number of Influenza A Positive cases in a region of Texas from the week of September 2, 2003, to the week of April 10, 2007. The number of cases peaks during the November-April period and is zero during the remaining weeks from late spring to early fall. Although medical data, this series shares many of the characteristics of the demand for certain spare parts. The number of flu cases and the demand for spare parts are "lumpy," with periods of activity followed by relatively long and variable periods of no activity. Can you think of any problems a series like the flu series might pose for ARIMA modeling? Can you think of a simple nonseasonal ARIMA model that might produce a reasonable forecast of the number of Influenza A Positive cases one week ahead?
Figure P-23
Time Series Plot of Weekly Influenza A Positive Cases

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Influenza A Positive Cases Week of Septerrber 2 2003-Week of April 10 2007 90 80- 70- 60- 50- 30 20- 10 1 21 42 63 84 105 126 147 158 189 Week