Easy, just try "123456" and "password" (or the top 10 of the the biggest password breachs at a given time), you will have a great hit ratio in a random sample of people.
Try reading Asimov's Foundation. You can't predict individuals, but given enough number of people, you will get a great trend hinting. And if the group you try to predict have some password culture on it (like this community), you should not be able to guess, even simple algorithms for passwords like the xkcd ones are pretty safe to random guessing.