the World Bank's International Comparison Program is of this type. Here the quantity data are updated each period from each of multiple countries, whereas the prices incorporated are kept the same for some period of time, e.g. the "average prices for the group of countries". Fisher index and Marshall–Edgeworth index The Marshall–Edgeworth index (named for economists Alfred Marshall and Francis Ysidro Edgeworth), tries to overcome the problems of over- and understatement by the Laspeyres and Paasche indexes by using the arithmetic means of the quantities: The Fisher index, named for economist Irving Fisher), also known as the Fisher ideal index, is calculated as the geometric mean of and : All these indices provide some overall measurement of relative prices between time periods or locations. Practical measurement considerations Normalizing index numbers Price indices are represented as index numbers, number values that indicate relative change but not absolute values (i.e. one price index value can be compared to another or a base, but the number alone has no meaning). Price indices generally select a base year and make that index value equal to 100. Every other year is expressed as a percentage of that base year. In this example, let 2000 be the base year: 2000: original index value was $2.50; $2.50/$2.50 = 100%, so new index value is 100 2001: original index value was $2.60; $2.60/$2.50 = 104%, so new index value is 104 2002: original index value was $2.70; $2.70/$2.50 = 108%, so new index value is 108 2003: original index value was $2.80; $2.80/$2.50 = 112%, so new index value is 112 When an index has been normalized in this manner, the meaning of the number 112, for instance, is that the total cost for the basket of goods is 4% more in 2001 than in the base year (in this case, year 2000), 8% more in 2002, and 12% more in 2003. Relative ease of calculating the Laspeyres index As can be seen from the definitions above, if one already has price and quantity data (or, alternatively, price and expenditure data) for the base period, then calculating the Laspeyres index for a new period requires only new price data. In contrast, calculating many other indices (e.g., the Paasche index) for a new period requires both new price data and new quantity data (or alternatively, both new price data and new expenditure data) for each new period. Collecting only