http://www.bls.gov/cpi/cpihqaitem.htm
>The CPI is calculated using prices for a fixed basket of goods and services through time. While the basket is periodically revised to reflect changing consumer expenditures, some items being priced in the sample come and go from the marketplace, making collection of these prices from month to month difficult. When an item is no longer available in the marketplace, a similar replacement item is selected. Often there are no similar items from which to choose, and as a result, a less comparable item is selected, potentially introducing quality change and an associated price differential into the index. The hedonic quality adjustment method removes any price differential attributed to a change in quality by adding or subtracting the estimated value of that change from the price of the old item.
That toilet paper you used to buy costs twice as much? Well the FED says the new toilet paper is of a higher quality, so it is actually counted as being lower priced then the toilet paper you bought for half as much last year.