The Short Term Trading Index was invented over 30 years ago by Richard Arms and is also known as ARMS Index. It is calculated by dividing weekly advancing issues by weekly declining issues and weekly advancing volume by weekly declining volume. The first result is then divided by the latter and the result is the TRIN.
If the index is above one, the average volume of stocks that fell on the NYSE was greater than the average volume of stocks that rose and vice versa. But it is most confirmative when it reaches extremes. This indicator rises sharply when the market is most depressed and selling is climaxing, and falls to very low levels during buying frenzies.
Bullish: Short Term Trading Index below 0 or extreme high value
Bearish: Short Term Trading Index above 1
Watch out for divergences between the market and that indicator