Machine Learning in Java - Second Edition

Sampling traps

Data collection may involve many traps. To demonstrate one, let me share a story. There is supposed to be a global, unwritten rule for sending regular mail between students for free. If you write student to student in the place where the stamp should be, the mail is delivered to the recipient for free. Now, suppose Jacob sends a set of postcards to Emma, and given that Emma indeed receives some of the postcards, she concludes that all of the postcards are delivered and that the rule indeed holds true. Emma reasons that, as she received the postcards, all of the postcards are delivered. However, she does not know of the postcards that were sent by Jacob, but were undelivered; hence, she is unable to account for this in her inference. ...