=+42. Monthly gas prices. We have seen that gas prices can fluctuate. But during some periods they have moved consistently. Here are the data extracted for one week of each month from January 2002 to May 2007.

300 250 200 150 2002 2003 2004 2005 Year 2006 2007 Price (cents/gal)

The bend in the plot and the increasing variation suggest a multiplicative model. Here is a multiplicative model with