Hi Poiseulle
The key to index maintenance is the adjustment of the divisor. Index maintenance should not change the level of the index. This is accomplished with an adjustment to the divisor.
Any change to the coins/assets in the index that alters the total market value of the index while holding coins/assests prices constant will require a divisor adjustment. This section explains how the divisor adjustment is made given the change in total market value.
This Equation shows the coin/asset being removed, coin “AB”, separately from the coins/assets that will remain in the index:
Index level
t-1= (Σ
ip
i*Q
i + P
ab Q
ab)/Divisor
t-1Note that the index level and the divisor are now labeled for the time period t-1 and, to simplify this example, that we are ignoring any possible ASF and adjustments to share counts. After coin/asset AB is replaced with coin/asset XY, the equation will read:
Index level
t= (Σ
ip
i*Q
i + P
xy Q
xy)/Divisor
tIn equations (3) and (4) t-1 is the moment right before coin/asset AB is removed from and XY is added to the index; t is the moment right after the event. By design, 𝐼𝑛𝑑𝑒𝑥 𝐿𝑒𝑣𝑒𝑙𝑡−1 is equal to 𝐼𝑛𝑑𝑒𝑥 𝐿𝑒𝑣𝑒𝑙𝑡 . Combining (3) and (4) and rearranging, the adjustment to the Divisor can be determined from the index market value before and after the change:
((Σ
iP
i*Q
i+P
abQ
ab) divisor
t-1 = Index level = ((Σ
iP
i*Q
i+P
xyQ
xy) divisor
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