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Topic: [ap-stat] RE: Variance
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Pat ballew

Posts: 455
Registered: 12/3/04
[ap-stat] RE: Variance
Posted: Oct 7, 2004 10:23 AM
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Maybe I can help... I think the reasoning is something like this... The
upper quartile is essentially the cut off for 75% of the cumulative data, so
it has standardized Z-score of about .674.... And by symmetry the z-score
for the lower quartile is the negative of this, or about -.674, so the
distance between the two quartiles (the IQR) is equal to about 2 (.674) or
about 1.35 standard deviations.

In this problem the IQR is a distance of ten units, so 10 = 1.35 Sigma...
And solving for sigma we get about 7.407... The variance is the square of
this, or about 55...

Hope I got that right, if not, delete this message before anyone sees it.


Pat Ballew
Lakenheath, UK

MathWords http://www.pballew.net/etyindex.html


-----Original Message-----
From: CONROY, LAURA Conroy [mailto://lconroy@ddschools.org]
Sent: Thursday, October 07, 2004 12:50 PM
To: for Teachers of AP Statistics
Subject: [ap-stat] Variance

On test 2C question #9, the question is: The five-number summary of the
distribution of scores on a statistics exam is: 0 26 31 36
50
316 students took the exam. The histogram of all 316 tests scores is
approx normal. The variance of test socres must be about: the answer
is 55
can someone explain this to me, please?



Laura Conroy
Mathematics Dept
DDHS
lconroy@ddschools.org

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