Chart from finance.yahoo.com. I would have expected a big uptrend after the first one, but it didn't really come. But after the second one, there was a downtrend, which I would have expected, too. Right? I'm kind of a noob to all of this. <img src="http://img.photobucket.com/albums/v326/ironfistx/spyhammers.gif">
If you go camping in the woods and sleep in a sleeping bag and come back with bugs sucking blood out of your leg, those are "bad ticks". As it relates to the market, "bad ticks" are prices that didn't actually take place. The first one where the low of the bar is ~126.7 probably should have been key-punched as 127.7. That would make the chart-bar look different. The second one later in the day at ~127.9 probably should have been key-punched as 126.9. Again, that would make a big difference on the chart. Since there was no volume spike when those bars were printed, they're probably "bad ticks" and not news releases nor fat-finger-itis.