If you are using old receipts, make sure that they have the date on them. Simply knowing that the prices listed are not current does not illustrate any real point. The change in CPI is only relevant if calculated for a specifically quantifiable amount of time.
Normally, the CPI is restricted to some of the most commonly used consumer items–foods such as milk and eggs, and others such as laundry detergent and shampoo. If you are using a record of your own purchases and are trying to determine the general trend in prices and not merely the change of a single item, you may want to exclude those items that are only occasional purchases.
If you are using a relatively small sample of items, you may be able to find the prices in flyers sent out by retail stores. It may be useful, for the sake of comparison, to make sure the prices used are based upon the same brands and from the same retailer. Because of the price differences at each store and from brand to brand, the only way to track the change of prices over time is to minimize these variables.
Think of the CPI as a percentage. Past prices represent a baseline, and that baseline is described as 100% of itself. Using the previous example, current prices would be 112. 5% of the previous prices.
Again, using the above example, the result would be 12. 5, representing a 12. 5% change in prices from the first period to the second. Positive results represent the rate of inflation; negative numbers reflect deflation (a rare fairly rare phenomenon in most of the world since the mid-twentieth century).
Also avoid comparing sale items. The official CPI calculated by the Federal Bureau of Labor Statistics uses a large number of items found in a variety of locations in order to eliminate short-term fluctuations. Calculating the change for individual items is still worthwhile, but sales are another variable that should be eliminated.
Using the example, the CPI would be 110.
