The arithmetic calculation involved in calculating SENSEX is simple, but problem arises when one of the component stocks pays a bonus or issues rights shares. If no adjustments were made, a discontinuity would arise between the current value of the index and its previous value. The Index Cell of the Exchange periodically adjusts the base value to take care of such corporate announcements.
■ Adjustments for Rights Issues: When a company, included in the compilation of the index, issues right shares, the market capitalisation of that company is increased by the number of additional shares issued based on the theoretical (ex-right) price. An offsetting or proportionate adjustment is then made to the Base Market Capitalisation (see ' Base Market Capitalisation Adjustment' below).
■ Adjustments for Bonus Issue: When a company, included in the compilation of the index, issues bonus shares, the market capitalisation of that company does not undergo any change. Therefore, there is no change in the Base Market Capitalisation, only the 'number of shares' in the formula is updated.
Other Issues: Base Market Capitalisation Adjustment is required when new shares are issued by way of conversion of debentures, mergers, spin-offs etc. or when equity is reduced by way of buy-back of shares, corporate restructuring etc.
■ Base Market Capitalisation Adjustment: The formula for adjusting the Base Market Capitalisation is as follows: New Base Market Capitalisation = Old Base Market Capitalisation X (New Market Capitalisation /Old Market Capitalisation) To illustrate, suppose a company issues right shares which increases the market capitalisation of the shares of that company by say, Rs.100 crores. The existing Base Market Capitalisation (Old Base Market Capitalisation), say, is Rs.2450 crores and the aggregate market capitalisation of all the shares included in the index before the right issue is made is, say Rs.4781 crores. The "New Base Market Capitalisation " will then be: Rs.2501.24 crores = 2450 X (4781+100)/4781 This figure of 2501.24 will be used as the Base Market Capitalisation for calculating the index number from then onwards till the next base change becomes necessary.
