Base effect

The base effect[1] relates to inflation in the corresponding period of the previous year, if the inflation rate was too low in the corresponding period of the previous year, even a smaller rise in the Price Index will arithmetically give a high rate of inflation now. On the other hand, if the price index had risen at a high rate in the corresponding period of the previous year and recorded high inflation rate, a similar absolute increase in the price index now will show a lower inflation rate now.

An example of the base effect:

The Price Index is 100, 150, and 200 in each of three consecutive periods, called 1, 2, and 3, respectively. The increase of 50 from period 1 to period 2 gives a percentage increase of 50%, but the increase from period 2 to period 3, despite being the same as the previous increase in absolute terms, gives a percentage increase of only 33.33%. This is due to the relatively large difference in the bases on which the percentages are calculated (100 vs 150).