Question 1104256
Here is a simple probability model for multiple-choice tests. Suppose that each student has probability p of correctly answering a question chosen at random from a universe of possible questions. (A strong student has a higher p than a weak student.) The correctness of answers to different questions are independent. Jodi is a good student for whom p = 0.77. 
How many questions must the test contain in order to reduce the standard deviation of Jodi's proportion of correct answers to half its value for a 100-item test?
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Find original standard deviation::
mean = np*100*0.77 = 77
std = sqrt(npq) = sqrt(77*0.23) = 4.104
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New conditions.  Find "n".
(4.104)/2 = sqrt(npq)
2.052 = sqrt(n*0.77*0.23)
4.211 = 0.1771n
n = 24 when rounded up
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Cheers,
Stan H.
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