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The most commonly used index formula is the Laspeyres index which measures the change in cost of purchasing the same basket of goods and services in the current period as was purchased in a specified base period.

Definition of The Fisher Index: The Fisher index is a price index, computed for a given period by taking the square root of the product of the the Paasche index value and the Laspeyres index value. (Econterms)

index developed by German economist Hermann Paasche for measuring current price or quantity levels relative to those of a selected base period. It differs from the Laspeyres index in that it uses current-period weighting.

http://www.britannica.com/eb/article?eu=59341