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**finitehelp****Member**- Registered: 2009-06-21
- Posts: 80

what is z-score?

A quart of Parker's milk contains a mean of 39 grams of butterfat, with a standard deviation of 2 grams. If the butterfat is normally distributed, find the probability that a quart of this brand of milk chosen at random will contain

a) between 39 and 43 grams of butterfat

b) more than 40 grams of butterfat

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**bobbym****Administrator**- From: Bumpkinland
- Registered: 2009-04-12
- Posts: 104,306

Hi finitehelp;

We'll start with a) first. 4 represents 2 standard deviations so it is .9772-.5 = .4772 is the probability that butterfat is between 39 and 43 grams. Tell me if you are having a problem here before we move on.

**In mathematics, you don't understand things. You just get used to them.****If it ain't broke, fix it until it is.** **A number by itself is useful, but it is far more useful to know how accurate or certain that number is.**

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**finitehelp****Member**- Registered: 2009-06-21
- Posts: 80

I have no idea how to solve this

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**bobbym****Administrator**- From: Bumpkinland
- Registered: 2009-04-12
- Posts: 104,306

what is z-score?

For instance with example a)

This site has much more than this as well as a diagram on the right of the standard normal curve which is the key to understanding statistics.

http://en.wikipedia.org/wiki/Standard_score

b) more than 40 grams of butterfat

1-.6915 = .3085 is the probability that a quart of buttermilk has more than 40 grams of butterfat.

For me to help further you must tell me what it is exactly that you do not understand. Look over the site I provided and if you have any specific question come back here. Also what textbook are you using?

*Last edited by bobbym (2009-07-19 03:06:01)*

**In mathematics, you don't understand things. You just get used to them.****If it ain't broke, fix it until it is.** **A number by itself is useful, but it is far more useful to know how accurate or certain that number is.**

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**finitehelp****Member**- Registered: 2009-06-21
- Posts: 80

I guess I would like to get a simpler explanation of z -score. I understand if you broke it down a simple as you could.

In the the second problem whats a real life example of when you would use this? I dont plan on making buttermilk soon but maybe some other example would make the concept clearer

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**bobbym****Administrator**- From: Bumpkinland
- Registered: 2009-04-12
- Posts: 104,306

Hi finitehelp;

Look at this site also:

http://www.measuringusability.com/zcalc.htm

They might provide an example that is clearer to you.

I too, find buttermilk uninteresting. I can provide a simplified coin toss problem that will demonstrate the statistics we have covered so far. Do you require it?

*Last edited by bobbym (2009-07-19 18:46:38)*

**In mathematics, you don't understand things. You just get used to them.****If it ain't broke, fix it until it is.** **A number by itself is useful, but it is far more useful to know how accurate or certain that number is.**

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