What technique is used to merge the prices & volumes of this overlap?
Any way you go it's going to have inaccuracies.
If you're trying to use the merged data to backtest a strategy, you're going to have a day there where you have a spurious big winner/loser (depending on long or short) when the price jumps as we switch from one contract to another.
A few solutions that occur to me are:
2) Just use the prices from whichever day has the highest volume and switch from old to new contract when the new contract's volume exceeds the old contract's volume. (Problem: loses some volume.)
3) Add the volumes together for the overlapped period and switch to new price at end of old contract.
4) Add the volumes together for the overlapped period and switch to new price when new contract volume exceeds old contract volume.
5) Add the volumes together for the overlapped period & do a weighted average on the prices. (Problem: Has more than one day of inaccurate prices.)
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