There are two types of error, systematic error, and random variance error; by repeating experiments you can average out and drive down the variance error.
This post is part of a series rerunning Eliezer Yudkowsky's old posts so those interested can (re-)read and discuss them. The previous post was Tsuyoku vs. the Egalitarian Instinct, and you can use the sequence_reruns tag or rss feed to follow the rest of the series.
Sequence reruns are a community-driven effort. You can participate by re-reading the sequence post, discussing it, posting the next day's sequence reruns post, or summarizing forthcoming articles on the wiki. Go here for more details, or to discuss the Rerunning the Sequences series.