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February 8th, 2010, 09:19 AM   #1
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Calculate probability value

Hello,

I found a page yesterday and i'm trying to figure it out how to calculate it like he did.

And before i continue, yes, formulas are there but i have no idea how to use them..

This is a part of article URL with formulas:
http://www.developer.com/design/arti...timization.htm

This is bigger picture of what i'm trying to calculate like he did:


This is the starting/raw data:
125/401 => 31.2% (point average for Original)
160/411 => 38.9% (point average for Combination 11)
143/453 = > 31.6% (point average for Combination 7)

Confidence level is 80%.

What i've tried and figured it out so far:

I know how to get "Sample proportions", that's an easy one
125 / 401 = 0,3117
160 / 411 = 0,3893

"Difference between sample proportions" is also simple:
You take the higher number:
0,3893 - 0,3117 = 0,0776

"Standard error of difference", no idea how to get that.

"z-value", I know, its:
0,0776 / 0,0334 = 2,325

But since I don't know how to calculate "Standard error of difference", getting
"z-value" is impossible.

And then, how on earth once you have all the data, do you calculate "Probability
value for the test".

I've probably spend like 5 hours so far searching the net for formulas and their explanation of "p" and similar and haven't found much that can calculate those two missing things.

Any help would be nice.
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