I'm not taking the bait. As far as I'm concerned, CPI methodology is a truism worthless by itself. Like saying "goods and services cost money". If you want to argue the specific numbers issued by the government are representative of the evolving purchasing power of the dollar, you have to do the work to prove it.
One of the more difficult problems faced in compiling a price index is the accurate measurement and treatment of quality change due to changing product specifications and consumption patterns. The concept of the CPI requires a measurement through time of the cost of purchasing an unchanging, constant-quality set of goods and services. In reality, products disappear, products are replaced with new versions, and new products emerge.
When a data collector finds that he or she can no longer obtain a price for an item in the CPI sample (often because the outlet permanently stops selling it), the data collector uses the CPI item replacement procedure to find a new item. Each priced item stratum in the CPI contains one or more ELIs. CPI commodity analysts have developed checklists that define further subdivisions of each ELI. When seeking a replacement in a retail outlet, the data collector first uses the checklist for the ELI to find the item sold by the outlet that is the closest to the previously priced item. Then the data collector describes the replacement item on the checklist, capturing its important specifications. The CA assigned to the ELI reviews all replacements and selects one of three methods to adjust for quality change and to account for the change in item specifications.
The following example describes the most common type of quality adjustment problem. Assume that a data collector in period t tries to collect the price for item j in its assigned outlet and is not able to do so because the outlet no longer sells this item. (A price for item j was collected in period t–1.) The data collector then finds a replacement item and collects a price for it. This replacement item becomes the new version v+1 of item j. The commodity analyst decides how the CPI treats the replacement. The commodity analyst has the descriptions of the two versions of item j. In addition, he or she has the t–1 price, , for the earlier version v and the period t price, , of the replacement version v+1. The following matrix displays the information available to the commodity analyst:
