There are three possibilities for the way open interest changes with each
trade in a futures market, depending on whether the long and short sides of
the trade are closing out already existing positions, or opening up new
positions.
If both sides are opening new positions, open interest goes up by the number
of contracts traded. If one side is opening a new position and the other is
offsetting an existing position then open interest is unchanged. If both are
offsetting existing positions then open interest is reduced by the number of
contracts traded.
The way I think of it is: every `trade' of a contract in a futures market
actually creates a completely new contract between the long and the
short side, whose initial present value is exactly zero for both parties. So in all
trades, the number of existing contracts goes up. It's not really that the
seller of a contract transfers that contract to the buyer. Instead the short
and the long make a new contract, and the seller thereby obtains a new short
position, which may or may not offset an already existing long position.
So, when determining how much money will flow at the end of the day from the
long to the short side of the market (or vice versa) it's not the total
number of contracts long and short that matters, but only the net number of
contracts on the long and short sides, those which have not so far been offset
by contracts in the opposite direction. That number is the open interest.
In the ES example, the open interest was 1541K in the March contract on the
prior day, and the volume was 1032K.
But we're missing an important piece of information before we can say what
happened: how much the open interest changed from the day before.
It's very unlikely of course, but in principle, if the open interest on the
day before were 509K, all 1032K of that volume would correspond to new
long and short positions that were opened up. If the open interest only
changed by a number much smaller than 1032K, then the situation is different.
trade in a futures market, depending on whether the long and short sides of
the trade are closing out already existing positions, or opening up new
positions.
If both sides are opening new positions, open interest goes up by the number
of contracts traded. If one side is opening a new position and the other is
offsetting an existing position then open interest is unchanged. If both are
offsetting existing positions then open interest is reduced by the number of
contracts traded.
The way I think of it is: every `trade' of a contract in a futures market
actually creates a completely new contract between the long and the
short side, whose initial present value is exactly zero for both parties. So in all
trades, the number of existing contracts goes up. It's not really that the
seller of a contract transfers that contract to the buyer. Instead the short
and the long make a new contract, and the seller thereby obtains a new short
position, which may or may not offset an already existing long position.
So, when determining how much money will flow at the end of the day from the
long to the short side of the market (or vice versa) it's not the total
number of contracts long and short that matters, but only the net number of
contracts on the long and short sides, those which have not so far been offset
by contracts in the opposite direction. That number is the open interest.
In the ES example, the open interest was 1541K in the March contract on the
prior day, and the volume was 1032K.
But we're missing an important piece of information before we can say what
happened: how much the open interest changed from the day before.
It's very unlikely of course, but in principle, if the open interest on the
day before were 509K, all 1032K of that volume would correspond to new
long and short positions that were opened up. If the open interest only
changed by a number much smaller than 1032K, then the situation is different.