Explanation The time reversal test is a test to determine whether a given method will work both ways in time, forward and backward. In other words of professor I. Fisher ''The test is that for formula for calculating the index number should be such that it will give the same ratio between one point of comparison and the other, no matter which of the two is taken as base. ∴ This tese is Time reversal test Key Points Unit test = This test requires that an index number formula should be such that it does not affect the value of the index number, even if, the units of the price or quantities quoted are altered. This test is satisfied by all the index formulas. Circular test = Another test of the adequacy of the index number formula is what is known as the 'circular test'. If in the use of index numbers interest attaches not merely to a comparison of two years, but to the measurement of price changes over a period of years. It is frequently desirable to shift the base. Factor Reversal Test = The factor reversal test requires that multiplying a price index and a volume index of the same type should be equal to the proportionate change in the current values (e.g. the “Fisher Ideal” price and volume indexes satisfy this test, unlike either the Paasche or Laspeyres indexes)