Paint the Tape
Painting the tape is the illegal practice of making a sale for the explicit purpose of changing the stock price. A fund manager might, for example, buy 100 shares of a stock at an unusually high price right before the market closes at the end of the month. Then he can report that his entire long position in that stock is worth that price. Brokers have even more control over the tape. If they have two orders they can buy the stock from one customer at one price then immediately sell it to another customer at a different price.
This practice works because traditional analysis gives so much weight to some prints while ignoring others. Most analysis gives a very high weighting to the last print before the close. This value can be manipulated, especially in low volume stocks and stocks with large spreads.
We use statistical analysis to avoid this problem. When we see one small print which does not correspond to the general trend, we throw that point away. This applies to explicit manipulation, like painting the tape, and to legitimate random noise in the market.
Although someone can request alerts based on the close price, this is the exception. Most of our alerts do not give strong weighting to any one price, and they do not act differently when the market closes. If there is a lot of volume after the market closes, we will continue to update the alerts. Additionally the user can filter the alerts to remove stocks trading on low volume or with large spreads. This allows the user to ignore stocks which are easily manipulated.