It may not be as obvious as you would think. The CPI was 207.917 in August and 208.490 in September. The 208.49 means that what cost $100 in 1983 now costs $208.49. Since 208.49/207.917 = 1.00276, prices were up .276%. If that happens for 12 months, the inflation rate would end up being 3.36%.
But that is based on just one month. In September 2006, the CPI was 202.9. That means that prices went up 2.76% in the last 12 months. In all of 2006, they were up 2.5%. So far in 2007, they are up 3.3%. The CPI was 201.8 in December of 2006. Since 208.49 (the Sept 2007 CPI) divided by 201.8 =1.033, we get a 3.3% increase. That works out to .364% per month. If we get that increase over 12 months, the inflation rate would be 4.4%.
How could they have gone up 2.76% over the last 12 months while they might go up 4.4% for all of this year? Prices fell in the last few months of 2006. The same thing happened in 2005. Both of those years we were headed to 4-5% inflation but prices fell the last few months to give us a decent rate for the year. The CPI was up only 3.4% in 2005. Will we get lucky the last few months this year?
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