# find number of Standard deviations from a given probability in a normal distributionC

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Thread starter 2 months ago
#1
Given a Probability of a given range about the mean of 0.95 I know that the number of standard deviations either side of the mean is 2.

How can I look up this value using a cg50. i.e given a particular probability use the CG50 to look up the number of standard deviations?
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2 months ago
#2
(Original post by nerak99)
Given a Probability of a given range about the mean of 0.95 I know that the number of standard deviations either side of the mean is 2.

How can I look up this value using a cg50. i.e given a particular probability use the CG50 to look up the number of standard deviations?
Not sure about the CG50, but on an fx-991EX you have to know that if a (symmetrical) range about the mean has area 0.95, then each tail has area 0.025. That make the area of the lower tail plus the central area equal to 0.975. Then you can use the inverse normal function with area = 0.975, SD = 1 and (mu) = 0 to give you the answer approx 2 standard deviations (1.96 to 3sf).
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2 months ago
#3
https://education.casio.co.uk/cg50-n...-in-stats-mode
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Thread starter 2 months ago
#4
(Original post by old_engineer)
Not sure about the CG50, but on an fx-991EX you have to know that if a (symmetrical) range about the mean has area 0.95, then each tail has area 0.025. That make the area of the lower tail plus the central area equal to 0.975. Then you can use the inverse normal function with area = 0.975, SD = 1 and (mu) = 0 to give you the answer approx 2 standard deviations (1.96 to 3sf).
Yes, as I said, I know the answer and how to find it but not with the CG50.
Last edited by nerak99; 2 months ago
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Thread starter 2 months ago
#5
Now that is a better CG50 site than the others I have seen so I leave it here as a link. Thankyou. As for the manual! Anyway, for future visitors. Given a probability boundary or 95% in a central area, the mean and SD of the actual distribution is irrelevant so what I did was ...

1. Menu [2] (stats)
2. DIST (f5)
3. NORM (f1)
4. InvN (f3)
5. Fill in the table.
6. DAta: Variable
7. At this point I can choose Tail as Right, Left or Central, in this case I choose CENTRAL (f3) as I am give '95%' is within a particular range.
8. Area 0.95
9. Variance 1
10. Mean 0
11. EXECUTE
I get given plus or minus 1.95996398 which is correct. Which for the estimate I was working with tells me that 95% of my data is within 2 standard deviations of the mean.

Comments on the transfer of statistics from tables to calculators are welcome.

BTW, I am not a fan of the new a-level chucking out a whole load of pure maths in order to force everyone to have to do S1 stats and M1 mechanics. Why not go the whole hog and have a buying vegetables section? Who makes these decisions?

Edit. it appears that the A-level Content Advisory Board,. was responsible. Entirely manned by University academics part from one senior grammar school teacher. Here https://alevelcontent.files.wordpres...-july-2014.pdf
Last edited by nerak99; 1 month ago
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