If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content

Introduction to random sampling

Can Sal predict the ratio of white to black balls without looking at all of them? Created by Brit Cruise.

Want to join the conversation?

Video transcript

- [Brit] Hey Sal. - [Sal] Hello Brit. - I have a challenge for you, 'cause I heard you have some kind of predictive powers, you're a bit of a magician. (Marbles Rattle) - There might be some truths to that, limited truth. (Laughing) - [Brit] I want to test it out. What I did was I filled this black bucket with a certain number of white and black beads. And I know what you're gonna say, I'm gonna prove to you that I have both white and black. - Okay. - So here's the white. - White. - No tricky business here, I'm a bit of a magician myself. - Math magician. - Math magician. And here's the dark. - Dark. - So they both exist. Now I'm wondering, you can touch it, you could do whatever you want, but you can't look inside of it. You can touch it. - I should scratch it. - Yeah, yeah, whatever you wanna do. - Tickle it. - Is there more white or black? That's my first question. Could you just guess of there's more or less and could you tell me - I refuse to guess. - [Brit] You won't even play? - 'cause I don't know, you just showed me two mar- I know there's at least one white and at least one black. - But I don't want to show you what's in here, so what- - So can I take some out? - Can I do it? - How about I do it? 'cause I think you might know the marbles or you might be looking in the thing when you pick it out. - Okay. - I want it to be randomly selected. - [Sal] Black. So can I do it again? - You can do it again. But we're gonna put the bead back in. - Okay, no, that's fair. (marbles rattle) - [Sal] Okay my eyes are closed. White. - [Sal] This is fascinating. So in 10 tries, I got five black and I got five whites. So, once again, I wouldn't bet my life savings on this, because once again, this might have been just the way I happen to pick it. But if I had to guess, I would say that you have an equal number of white and black. - So that's your guess and your ratio? - Well I would say one to one, for every white there's a black. I would guess whatever, half of those are white and half of those are black. Now, I could very well be wrong. Okay, show us what's going on. - I got you there. We have two and four. - Oh. So the correct one is, wow I had two whites for every black. The ratio was 2:1. - So at the beginning, you were actually very close. - I was spot on in the beginning. - And that's why I think the more trials, perhaps less likely that you're gonna get it correct. - I disagree with that. I disagree with that. I think that the more trials, it just happened in that it kind of took me off the right path this time. But I think if I did that more times than not, it would get me closer to the right answer.