The TRIN indicator ( also known as the Arms Index, after Dick Arms ) is calculated as the following on the NYSE: (Advancing Issues / Declining Issues) divided by (Advancing Volume / Declining Volume). Values above 1.00 indicated that there is selling in the market. Values under 1.00 indicate that there is buying in the market. 25 years ago I would run the TRIN ( or TRIQ for the NASDAQ ) in a 10-day moving average to figure out how "overbought" or "oversold" the market was. The problem with this is that the index can be susceptible to some VERY large volume stocks that wind-up on the Most Active List and are either +1/8th or -1/8th. It's basically a reflection of Overbought/Oversold much like a "Slow Stochastic Oscillator". Some sort of a stochastic oscillator or even the TICK might be more productive for day-traders and scalpers. Hope this helps.