Standard Deviation

helpmepleasestats

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So, I have a distribution with a mean of 75. I was given an upper limit of 80. No more than 2% can exceed this number. How do I calculate the SD?

(A previous problem had a mean of 75 and a standard deviation of 4.2 with an upper limit of 80. 11.7% of the datum exceeded 80, and the z-score of that percent was 1.1905)

I'm thinking, so far, that I would use the ZDMS formula.

Z = I know the 2%, so I think I would make that in to a Z-score, but I don't know how
D = 80
M = 75
S = Need to know
 
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So, I have a distribution with a mean of 75. I was given an upper limit of 80. No more than 2% can exceed this number. How do I calculate the SD?

(A previous problem had a mean of 75 and a standard deviation of 4.2 with an upper limit of 80. 11.7% of the datum exceeded 80, and the z-score of that percent was 1.1905)

I'm thinking, so far, that I would use the ZDMS formula.

Z = I know the 2%, so I think I would make that in to a Z-score, but I don't know how
D = 80
M = 75
S = Need to know
You need to look into the table of the normal distribution, and find what value of z gives a cumulative 98% (or a single-ended tail of 2%). That number of standard deviations above the mean is equal to (80-75).
 
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