You are Here: Home >< Maths

# Simple variance question. Watch

1. The question is : it is believed that a treatment has chance pi = i/10 of success for patient i i= 1, : : : , 5.
Let T be the total number of patients in the trial for whom the treatment is
successful. Assuming that the outcomes for patients are independent
compute E(T) and Var(T)?

Applying Var(T)=E(T^2)-((E(T)^2) gives me the wrong answer.

I do not understand why this is.
Thanks alot for any help !!!
2. (Original post by rainbowsss)
Applying Var(T)=E(T^2)-((E(T)^2) gives me the wrong answer.[/B]

I do not understand why this is.
Thanks alot for any help !!!
Unless I made a slip, I get Var(T)=0.75 with that formula.
3. Oh okay thanks. I think my trouble is calculating E(x^2) which I get to be 1+4+9+16+25/10. Cheers.
4. (Original post by rainbowsss)
Oh okay thanks. I think my trouble is calculating E(x^2) which I get to be 1+4+9+16+25/10. Cheers.

And you need to expand that first.

Note:

In full

and with just the relevant part.

as they're independent.
5. Oh okay, so as in E(T1^2+T1T2+T1T3+T1T4+T1T5+...+T 4^2+T4T5+...+T5^2) = E(T1^2)+....+E(T5T4)+E(T5T5),
where e.g E(T5T5)=25/100 , E(T1T2)=2/100?
Cheers.
6. (Original post by rainbowsss)
Oh okay, so as in E(T1^2+T1T2+T1T3+T1T4+T1T5+...+T 4^2+T4T5+...+T5^2) = E(T1^2)+....+E(T5T4)+E(T5T5),
Don't forget, cross terms, e.g. T4T5 will occur twice.

where e.g E(T5T5)=25/100 , E(T1T2)=2/100?
Cheers.
E(T5T5) = 5/10 - see my previous post.

TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Updated: May 12, 2013
Today on TSR

### Medicine offers rolling in

Which unis have sent theirs?

### I hate Christmas

Discussions on TSR

• Latest
• ## See more of what you like on The Student Room

You can personalise what you see on TSR. Tell us a little about yourself to get started.

• Poll
Useful resources

### Maths Forum posting guidelines

Not sure where to post? Read the updated guidelines here

### How to use LaTex

Writing equations the easy way

### Study habits of A* students

Top tips from students who have already aced their exams

## Groups associated with this forum:

View associated groups
Discussions on TSR

• Latest
• ## See more of what you like on The Student Room

You can personalise what you see on TSR. Tell us a little about yourself to get started.

• The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE

Reputation gems: You get these gems as you gain rep from other members for making good contributions and giving helpful advice.