The geometric mean of the base‐weighted or Laspeyres index and the current-weighted index (Paasche index) for the same period. Irving Fisher (1867–1947), an American economist, labelled it as ideal because it has the property of consistency across two periods: if the value of the index between periods s and t is I_st then the value of the index in the reverse direction from t to s satisfies I_ts = 1/I_st. See also Drobisch price index.