Need help with a statistics problem

[RedArmy]

Alright, so the basically I have a log-normal distribution with a mean of 900 and std dev. of 725. It wants me to give the values for mu and sigma. Since I know E(x) and Var(x) I used the formulas found on the Wikipedia page under 'log-normal distribution'. However, after plugging in the two numbers I get an answer of 6.80 (basically just the value of ln(900)) while the book has an answer of 6.507 for mu.

I tried doing the problem the other way by using the separate E(x) and Var(x) equations and substituting and canceling variables but I still get the same answer. The sigma answer is supposedly .769 but I don't really have a way to confirm that due to the fact that I can't get the other value. If you have any insight into this problem I'd appreciate it, thanks!

 

[RedArmy]

 

[homercles337]

I think youre very confused. What are you trying to do?

 

[frostedflakes]

There are different methods for calculating mean and variance, depending on whether the variable is discrete or continuous. Maybe this is what's throwing you off?

 

[RedArmy]

Originally posted by: homercles337
I think youre very confused. What are you trying to do?

No, I'm pretty confident that I'm doing it the right way considering the book told me what formula to use. The formula is: E(x)=e^(mu + (sigma^2)/2), however that alone can't help me find the answer since I have 2 unknown variables.

I'm confused one how I'm supposed to go from the information given to the answers when the book is only giving me one equation. That's why I've been using the alternate equations that are provided on the Wikipedia page to try and arrive at the intended answer.

Originally posted by: frostedflakes
There are different methods for calculating mean and variance, depending on whether the variable is discrete or continuous. Maybe this is what's throwing you off?

I guess that could be a possibility, but I'm pretty confident in what I should be using. We just started this material and our professor is...less than adequate when it comes to explaining stuff.

 

[Eeezee]

Originally posted by: RedArmy

Originally posted by: homercles337
I think youre very confused. What are you trying to do?

No, I'm pretty confident that I'm doing it the right way considering the book told me what formula to use. The formula is: E(x)=e^(mu + (sigma^2)/2), however that alone can't help me find the answer since I have 2 unknown variables.

I'm confused one how I'm supposed to go from the information given to the answers when the book is only giving me one equation. That's why I've been using the alternate equations that are provided on the Wikipedia page to try and arrive at the intended answer.

Originally posted by: frostedflakes
There are different methods for calculating mean and variance, depending on whether the variable is discrete or continuous. Maybe this is what's throwing you off?

I guess that could be a possibility, but I'm pretty confident in what I should be using. We just started this material and our professor is...less than adequate when it comes to explaining stuff.

That's because statistics is a horribly ill-understood concept. Statistical mechanics is even less understood.

 

[frostedflakes]

Can you give us the whole problem w/all the information it gives you, maybe someone can mess around with it and figure it out.

EDIT: Also something I just thought about, make sure you're using the correct base. Are you maybe using base e when it should be base 10?

 

[homercles337]

Originally posted by: frostedflakes
Can you give us the whole problem w/all the information it gives you, maybe someone can mess around with it and figure it out.

Exactly, OPie is very confused.