  Human decision making by professionals trading daily in the stock market can
be a daunting task. It includes decisions on whether to keep on investing or to
exit a market subject to huge price swings, and how to price in news or rumors
attributed to a specific stock. The question then arises how professional
traders, who specialize in daily buying and selling large amounts of a given
stock, know how to properly price a given stock on a given day? Here we
introduce the idea that people use heuristics, or "rules of thumb", in terms of
"yard sticks" from the performance of the other stocks in a stock index. The
under- /over-performance with respect to such a yard stick then signifies a
general negative/positive sentiment of the market participants towards a given
stock. Using empirical data of the Dow Jones Industrial Average, stocks are
shown to have daily performances with a clear tendency to cluster around the
measures introduced by the yard sticks. We illustrate how sentiments, most
likely due to insider information, can influence the performance of a given
stock over period of months, and in one case years.
