Indices are calculated at a fixed interval rate for the duration of a trading session. However, the first values of an index may not reflect accurately the market status, because some constituent stocks of that index may have not have had yet any trades, and the pre-open stock price taken in this case may be quite different from the opening stock price. This may lead to a misleading figure.

For this reason, the opening value shown in the historic diagrams is calculated as follows: for all constituent stocks that have had trades during the first five minutes of a trading session, the price of the first such trade is taken as the opening value; for other stocks, the adjusted previous close price is taken as the opening price. Then, the value of the index is calculated with the usual calculation formula, by summing the above opening prices of constituent stock and dividing the sum by the basis of the index.

The index opening price thus calculated serves only for displaying the historical diagrams and does not constitute official data, nor is it available via the Helex systems or products for download and processing.