Before comparing, make sure both values (your previous and current values) refer to the same product. If one is better in some way, your costs are not directly comparable. [2] X Research source
In these cases, you will need to do some online research to locate previous cost figures. Try typing in the item and year you are interested in, along with “cost” or “value,” to locate cost information for that time. For example, cost information for various consumer goods from 1900 until the present can be found at http://mclib. info/reference/local-history-genealogy/historic-prices/.
For example, if you bought a gallon of milk a month ago for $2. 50 and, today, it is $3. 50, subtract $2. 50 from $3. 50 to get the dollar amount of the change, or $1. 00 in this case.
In the example, this would be $1. 00 (the result of the previous step) divided by $2. 50 (the previous cost). The result is 0. 40, which is expressed as a number rather than a dollar amount.
In the example, this is 0. 40×100{\displaystyle 0. 40\times 100}, which is 40 percent. So, the cost of a gallon of milk has increased by 40 percent between the previous and new costs.
