New reference year

Why change the index reference year?

The reference year of an index serves as the reference period for describing the price development relative to the reference year. Changing the reference year of an index is therefore a way of making the reference period and thus the comparisons of the price development more relevant. 

What is the impact of changing reference years for published series?

First, changing reference year means that the index level is converted. Index numbers are converted to a new reference year, and index numbers with a different reference year cannot be directly compared to the converted indices. 

Statistics Norway publishes price indices with one decimal. This also means that all published rates of change including monthly change, twelve-month change and annual change are calculated based on indices with one decimal. Published rates of change thus correspond to the published index levels. 

It is particularly important to note that the change of reference year may give rise to different rates of change compared to those already published. This means that rates of change referring to 2015 = 100 may differ from those calculated with a reference year 2000 = 100. This is mainly caused by effects of rounding index numbers, as index numbers are converted to 2015 = 100 with indices with more than one decimal. This is done to convert the indices to the new reference year in a neutral manner. For PIF, which has published indices back to 1929, there will be considerable differences in published rates of change due to the change of reference year. This is on the one hand because index numbers and rates of change are now calculated and rounded consistently with one decimal throughout the PIF time series. In addition, differences in rounded index numbers will have a greater impact on rates of change based on indices that date further back in time which are at a considerable lower level compared to indices in recent periods. 

Converting between reference years

All retrospective indices for the PPI is converted referring to 2015 = 100 to ease comparisons of the price development for users. In some situations, however, it could be still be necessary to continue using indices with 2000 = 100. The example below demonstrates how indices with 2015 = 100 can be converted back to 2000 = 100 for a given index in January 2019:

Average index year 2000 (2015=100): 44,5

Index per January 2019 (2015=100): 116,3

Converted index with 2000=100 per January 2019: 116,3/44,5*100=261,3

For questions related to the change of reference year please contact produsentpris@ssb.no.