Nikkei 225 is a stock market index for the Tokyo Stock Exchange (TSE). It has been calculated daily by the Nihon Keizai Shimbun (Nikkei) newspaper since 1950. It is a price-weighted average (the unit is yen), and the components are reviewed once a year.
Currently, the Nikkei is the most widely quoted average of Japanese equities, similar to the Dow Jones Industrial Average. In fact, it was known as the "Nikkei Dow Jones Stock Average" from 1975 to 1985.
The Nikkei 225 began to be calculated on September 7, 1950, retroactively calculated back to May 16, 1949.
Currently, the index is updated every 15 seconds during trading sessions.
Weighting and modifications
Stocks are weighted on the Nikkei 225 by giving an equal weighting based on a par value of 50 yen per share. Events such as stock splits, removals and additions of constituents impact upon the effective weighting of individual stocks and the divisor. The Nikkei 225 is designed to reflect the overall market, so there is no specific weighting of industries.
Changes to the components
Stocks are reviewed annually and announcements of review results are made in September. Changes, if required, are made at the beginning of October. Changes may also take place at any time if a stock is found to be ineligible (e.g., delisting). All proposed changes will be announced in Nikkei's Japanese newspapers and will appear on NNI.
After a stock has been replaced, the divisor is reviewed and modified to ensure a smooth transition of the stock index.