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According to the source, the index is relative to New York City, which means that each index should be 100 percent. If another city has, for example, a rent index of 120, it means that, on average, rents in that city are 20 percent pricier than in New York City. If a city has a rent index of 70, that means that the city rents are 30 percent less expensive than in New York City. 

The local purchasing power index is in relation to New York City, with NYC's index score being 100. The source states the following: “Local purchasing power shows relative purchasing power in buying goods and services in a given city for the average net salary in that city. If domestic purchasing power is 40, this means that the inhabitants of that city with an average salary can afford to buy on average 60% fewer goods and services than New York City residents with an average salary.”

The source did not provide a release date. Date given here is the access date.