Math Is Fun Forum

kirha

Member

Registered: 2006-11-27

Posts: 1

statistics and probability

I really need some help with this

Assume you have a die that is made in a way that the probability to get "1" is 1/2, while the probability to get 2-3-4-5 and 6 is equal. X marks the nuber of eyes that you get when it is tossed.

Find the probability distribution!?
Find E(X)
Find Var(X)

Thank you for any help

Last edited by kirha (2006-11-27 12:10:52)

fgarb

Member

Registered: 2006-03-03

Posts: 89

Re: statistics and probability

Here's a strategy:
1) First figure out what the probability is of rolling each number on the die (this shouldn't be too tough).
2) Then, predend that you rolled the die a thousand times (or whatever number you want). Figure out how many times you can expect to roll each number based on the probabilities you got in 1. Make sure you don't round away fractions!
3) You can directly use the results of 2 to plug into the formulas for E[x] and V[x], the expected mean and standard deviation.

mathsyperson

Moderator

Registered: 2005-06-22

Posts: 4,900

Re: statistics and probability

I don't see the point of Step 2 in that. Using the probabilities that were found in 1) would be more sensible.

But the other steps are right. Once you've found the probability distribution, working out E(X) and Var(X) shouldn't be too much of a problem.

---

Why did the vector cross the road?
It wanted to be normal.

fgarb

Member

Registered: 2006-03-03

Posts: 89

Re: statistics and probability

Definitely true - step 2 is entirely optional. I just find it easier to think about things and explain them in terms of performing thought experiments to see what outcome you'd get.