One of the less well understood statistics in the basic daily trading data set is open interest. It’s a statistic that is generally included in end-of-day price quotes along with volume, but a lot of traders tend to ignore it, potentially to their detriment. Open interest is the total number of open contracts in futures and options trading. Recall that in derivatives markets like futures and options there must always be matching longs and shorts. For every mini S&P 500 futures contract that I am long, there is a short...