My roommate pointed out a couple of weeks ago that I always get Chinese takeout for dinner; however, we never get home at the same time, and most days, she's not even in the apartment when I arrive. How could she, a very bright and educated individual, come to such a conclusion after seeing so little data?
In fact, by my count, she only saw me bring home Chinese takeout twice before she decided that yes, I do in fact eat Chinese every single day of the week. In reality I rotate through four choices -- sandwiches, Japanese, pizza, or Chinese with a few ventures out every now and then. This week I've had Japanese, hot dogs, Mediterranean twice, sandwich, burger, and Chinese.
This is one of the reasons we need Statistics. What we perceive isn't always the truth. I might have had Chinese takeout on Monday and Friday, but do you know what I had on the days in between? If no, can you make an educated guess?