Bucket's Questions

[bucket]

ah cool.
thanks man =]

[bucket]

in normal distrubution; are the probabilities and going to be the same, if the mean and the standard deviation are the same?

[shinny]

Yeh they are, since

[bucket]

thanks a lot man =]

[bucket]

Say that I have a normal distribution function with mean 40 and standard deviation 5.
If I want to find c for , I go into the stat/list editor, inverse>normals and type in the mean sd and the area 0.75. What do I do if I want to find ?

[shinny]

and then just do what u've been doing all along as usual =T

edit: oh, and to know that symmetry property, just draw yourself a graph of a normal distribution and its quite easy to work out why its the case, as well as working out some of the harder symmetry properties such as how to interpret Pr(a<Z<b) etc.

[bucket]

thanks a shitload man!
good explanations

[bucket]

"The weight if cats is normally distributed. It is known that 10% of cats weigh more than 1.8kg and 15% of cats weigh less than 1.35kg. Find the mean and the standard deviation."

[Collin Li]

Hint (and this is how I analyse the question):
A normal distribution (like the binomial distribution) is always defined by two parameters. Here, you have two pieces of information, so you should be able to find the two unknowns. The question is, how do I get these two expressions into a form that involves the mean and the standard deviation?

Try using this:



So you will get:





Use invNorm(:

= invNorm(k), where

Hence,

invNorm(0.90) =                     (1)

invNorm(0.15) =                 (2)


(1) - (2) yields:

[bucket]

hmm you're right but I'm confused . what is invNorm?
When I do inverse normals I use a program where I have to enter the area(probability), the standard deviation and the mean. :S

edit**
I worked it out =]. Thanks a lot Coblin.

[excal]

Hint (and this is how I analyse the question):
A normal distribution (like the binomial distribution) is always defined by two parameters. Here, you have two pieces of information, so you should be able to find the two unknowns. The question is, how do I get these two expressions into a form that involves the mean and the standard deviation?

Try using this:



So you will get:





Use invNorm(:

= invNorm(k), where

Hence,

invNorm(0.90) =                     (1)

invNorm(0.15) =                 (2)


(1) - (2) yields:

coblin:

[Collin Li]

coblin:

Thanks, typo.

Only the formula needs to be fixed.

[/0]

When do you use capitals and when don't you?

You see in the Essential Meth book it gives instead of .

Also do you think any examiners will be pedantic enough penalise us for this?

[Collin Li]

When do you use capitals and when don't you?

You see in the Essential Meth book it gives instead of .

Also do you think any examiners will be pedantic enough penalise us for this?

They won't be pedantic enough.

I think you should always use capitals. The convention is that capital letters denote random variables. "Z" and "X" are both random variables.

[/0]

Thanks coblin... hmm, so the small letters actually stand for certain numbers and the large ones can stand for a set of numbers, that's alright... except, we don't write normal equations like . Oh well I guess I'm the one being pedantic now...