Index conversion function

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Index conversion function (ICF) is a variable describing how one index can be converted to another in a particular context. The variable is indexed by the two indices, and the values in cells represent the fraction of the total amount of substance (e.g., area, population) in the union determined by the two indices. For example, a country may be treated as a whole, or it may be divided into (i.e., indexed by) counties or municipalities. In this case, the three indices are "crisp" in the sense that a municipality belongs to exactly one county, and a county belongs to exactly one country. There can also be "overlapping" indices. For example, for a given population, a certain fraction of (not all) men or women belong to a particular age group. If the result domains of the two indices are not the same, the fraction of substance that goes beyond each index must be specified.

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NOTE! The ICF can also be operationalised in a way that instead of fractions, the values are given in substantive units (e.g. surface area for spatial indices, population size for populations). Then the value must be divided by the total sum of the variable to get proper fractions.