Question

Last hour - how to calculate standard deviation when I'm given the 95%th data, mean and it is a normal distribution?

Answer 1

if no other information given assume mean of 0 and sd of 1

Answer 2

are you given a zscore?

Answer 3

95th percentile you say?

Answer 4

I thought
mean + 2SD = 95%th data. Not sure though
Yup. sorry.. I don't really how to express it :(

Answer 5

\[z=\frac{x-\bar x}{\sigma }\]
\[\sigma =\frac{x-\bar x}{z }\]

Answer 6

i thought z is the standard score?!

Answer 7

it is, it is the measure of how many sds you are from the mean

Answer 8

is the 95% spread across the mean? or is it a measure from the left tail?

Answer 9

but yeah, 95% of the data falls within 2sd from the mean

Answer 10

Hmm,.. like 95% of the students score below 80. the mean is 70 ( sorry is the data are strange. I just make them up)

Answer 11

but without knowing any of the particulars, you cant really tell how large the sd is

Answer 12

But it is a normal distribution....

Answer 13

80-70
------ = z ; such that ztable relates it to .9500
sd

Answer 14

the z is like 1.96 i think ...

Answer 15

But with 2 unknowns.. how can you find them?

Answer 16

z is known by the 95%

Answer 17

when z=1.645 ; the area under the cure is 95%

Answer 18

http://www.google.com/imgres?q=ztable&hl=en&sa=X&biw=1280&bih=923&tbm=isch&prmd=imvns&tbnid=Jxx_0ZdxothsZM:&imgrefurl=http://statstutorstl.blogspot.com/2010/07/z-table-gives-probabilty-distribution.html&docid=zClDTy0ZRtvlpM&imgurl=http://4.bp.blogspot.com/_5u1UHojRiJk/TEh9BHxxPUI/AAAAAAAAAIQ/DafeQNMYFoE/s1600/ztable.gif&w=756&h=1109&ei=sU6LT_ikG8OXtwePvc3hCQ&zoom=1&iact=hc&vpx=334&vpy=167&dur=123&hovh=272&hovw=185&tx=94&ty=154&sig=115974570052349639520&page=1&tbnh=161&tbnw=110&start=0&ndsp=26&ved=1t:429,r:1,s:0,i:73

Answer 19

o.o .... the percentile is the standard score?!

Answer 20

that depends on what amount of information you are looking for; if 95% score 80 or less than you are looking for the area under the curve from the left tail

Answer 21

if your looking for 95% of the scores that are about the mean you have to split the area