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Topic: TI-83 and the normal approximation
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John Magee

Posts: 40
Registered: 12/6/04
TI-83 and the normal approximation
Posted: Feb 5, 1997 10:35 AM
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As we were covering IPS Ch. 5 and discussing the normal approximation to the
Binomial. One of my students did a problem much differently than I and then
asked a very pertinent question. Why do we need the normal approximation.
Using Chapter 5 - Problem 4 from Test Bank for IPS from McCabe and McCabe
"Offspring of a particular genetic cross have an undesirable trait with
probability 1/8. Inheritance of this trait by separate offspring is
independent. You examine 100 offspring from this cross and count the number
X who have the undesirable trait. What is the probability that 10 or less of
the offspring have this trait?"

Using the TI-83 normal approximation you would get .2248 from
normalcdf(-999,10,12.5,sd) with the continuity correction you would get .2727
from normalcdf(-999,10.5,12.5,sd). For those of you unfamiliar with TI-83
normalcdf(lowerbound, upperbound, mean, sd) returns area under curve quickly.

The student had done simply binompdf(100,0.125) stored to L1 and then did
sum(L1,1,11) and arrived with .280999. TI-83 binompdf(100,0.125) returns
all probabilities from 0 to 100 for p = 1/8. He summed items 1 to 11 to get
P(0) through P(10). He was, with ease at which he had done this, amazed
that anyone would do it the other way.
I was wondering if anyone else had had this come up in class, and wondered
if AP graders are aware of this binomial summing ability.

I have really enjoyed all the posts, I have been reading since late
December, and I thought it was time for me to share an idea or two.

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