Rule of Inventory like any other commodities (crude oil, bmetals) are same. More additions means there's more storage of these commodities to their respective warehouse, and therefore more addition tends to be bearish for the commodity and vice versa. But caution is required while trading immediately after the inventory data hits the market as lot of speculation is already at work and you may see price doing exactly opposite to this thumb rule. But to get a long term picture its exactly fine. You have to see the long term trend of past inventories, data of which is freely available, plus you have to add the seasonality factor to see why at some time inventory is negative or positive, for example natural gas inventory in past few weeks were negative and to the lowest due to below normal temperature in the U.S which is the major consumer. For coming month as summer starts there you may see inventory in positive figures like 100+ weekly additions, which mean less consumption and like wise.
