The index is taken from the cost of a standard "basket of goods" that is purchased in each location. The numbers compare mean prices in the base city to mean prices in the host city. This comparison is not biased in any direction and provides a good cost comparison for the cost of goods in two different locations for a middle-to-high income earner, especially in cities that have similar standards of living (such as the cities included in the table). It is worth noting that the index is reversible. For example, in the numbers provided, Dublin=100 while London=124, where Dublin is the base city (in other words a person from Ireland living in London). If London is the base city (and therefore equal to 100), then Dublin would be 76. In comparing Dublin to New York with Dublin as the base city, New York is 117; where the cities are reversed and New York becomes the base at 100, Dublin becomes 83.