Theoretical probability distribution example: tables
About this video
Two 3-sided dice are rolled and D is the non-negative difference of the numbers. A 3 by 3 table shows all 9 equally likely outcomes, giving P(D=0) = 3/9 = 1/3, P(D=1) = 4/9 and P(D=2) = 2/9. This is the theoretical probability distribution of D.
You'll be able to
- use a table to list all outcomes of rolling two dice.
- find the value of a random variable for each outcome.
- build a theoretical probability distribution by counting outcomes.
Key words
- theoretical probability distribution
- A distribution found by reasoning about equally likely outcomes, not by running experiments.
- non-negative difference
- The larger number minus the smaller number, so the result is never below zero.
- equally likely outcomes
- Outcomes that each have the same chance of happening.
Transcript
- [Instructor] We're told that a board game has players roll two 3-sided dice, these exist and actually I looked it up, they do exist and they're actually fascinating. And subtract the numbers showing on the faces. The game only looks at non-negative differences.
For example, if a player rolls a one and a three, the difference is two. Let D represent the difference in a given roll. Construct the theoretical probability distribution of D. So pause this video and see if you can have a go at that before we work through it together.
All right, now let's work through it together. So let's just think about all of the scenarios for the two die. So let me draw a little table here. So let me do it like that and let me do it like this. And then let me put a little divider right over here. And for this top, this is going to be die one and then this is going to be die two.
Die one can take on one, two, or three and die two could be one, two, or three. And so let me finish making this a bit of a table here. And what we wanna do is look at the difference but the non-negative difference. So we'll always subtract the lower die from the higher die.
So what's the difference here? Well, this is going to be zero. If I roll a one and a one. Now, what if I roll a two and a one? Well, here the difference is going to be two minus one, which is one. Here the difference is three minus one, which is two. Now what about right over here?
Well, here the higher die is two the lower one is one, right over here. So two minus one is one, two minus two is zero. And now this is gonna be the higher roll, die one is gonna have the high roll in this scenario. Three minus two is one. And then right over here, three minus one is two.
Now die one rolls a two, die two rolls a three. Die three is higher, three minus two is one. And then three minus three is zero. So we've come up with all of the scenarios and we can see that we're either gonna end up with a zero or one or a two when we look at the positive difference.
So there's a scenario of getting a zero, a one or a two. Those are the different differences that we could actually get. And so let's think about the probability of each of them. What's the probability that the difference is zero. Well, we can see that one, two, three of the nine equally likely outcomes, result in a difference of zero.
So it's gonna be three out of nine or one-third. What about a difference of, let me use the blue, one? Well, we could see there are one, two, three, four of the nine scenarios have that. So there is a four ninths probability. And then last but not least a difference of two.
Well, there's two out of the nine scenarios that have that. So there is a two ninths probability right over there. And we're done. We've constructed the theoretical probability distribution of D.
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