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[apstat] RE: Variance
Posted:
Oct 7, 2004 10:23 AM


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 Zscore of about .674.... And by symmetry the zscore 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: [apstat] Variance
On test 2C question #9, the question is: The fivenumber 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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