From Investors' Intelligence Sentiment Indicator: 

The “Investors Intelligence Survey“ or IIS is one of the oldest weekly sentiment indicators used today.  Charts of the Investors’ Intelligence Survey (IIS) “Bulls over Bulls plus Bears” versus the market are key tools for stock market technical analysis or sentiment.  The IIS began in January 1963 by A.W. Cohen and has been published every week ever since.

The IIS questions stock-market newsletter writers once a week to see if they were bullish or bearish on the stock markets in the near-term. Newsletter writers have a large following as a group and are thus considered “market experts.“  Cohen thought he could survey these "experts" to find out when it was best to be long or short the markets.  Before he started, Cohen expected the best time to be long would be when the most advisers were bullish. Cohen was surprised to find out that the newsletter writers, just like average investors and speculators, were usually wrong at major turning points!

Cohen found the best time to go long the stock markets was not when the most advisers were bullish, but when they were bearish! Cohen’s IIS survey turned out to be a wonderful contrarian indicator!