Fisher Ideal Index

The Fisher ideal index is a superlative index number that aggregates many individual prices or quantities into a single measure of overall change by taking the geometric mean of the Laspeyres (base-weighted) and Paasche (current-weighted) indices. Proposed by Irving Fisher in his 1922 treatise as the 'ideal' formula because it passes a battery of desirable axiomatic tests, it was later shown by W. Erwin Diewert to be exact for a flexible (quadratic) aggregator, giving it both an axiomatic and an economic-theoretic justification. It is the index of choice when a measure must satisfy the time-reversal and factor-reversal tests exactly.