normal distribution SD question long, question, practice, problem

ashycal

on a practice problem I am having trouble with…the answer says 99% of observations fall w/in 3 SD of the mean. I had thought 99% of the observations fall w/in 2.575 SD of the mean…? Please someone clarify, it has been a very long day

btcapital

pg441
The key word is “Approximately”:
Approximately 68% fall in +1SD
Approximately 95% fall in +2SD
Approximately 99% fall in +3SD

Flok

I thought that was a garbage question, too. I’ve memorized 90/1.65, 95/1.96, 99/2.58 and that’s what I’m using on the exam.

agulani

SD___________________Percentage in CI
–
1_______________________68.27
1.645___________________ 90%
1.96___________________ 95%
2 _____________________ 95.45%
2.576 ___________________ 99%
3 _____________________ 99.73%
4 _____________________ 99.99%

Finalnub

there is a graph that says something like 99.7% of observations (for a norm dist) fall w/in + 3 SD of the mean…
it just confused me bc a lot of the quant questions refer to the use of 99% conf interval and + 2.575…
the .7% must be represented by the .425 SD difference …
hopefully that helped a little bit.

dece2011

someone want to clarify this…dont have the book handy right now. Thanks

ramzes

Thanks njlevel10610. how did the PM mock go for you?? I dont know if I was just tired or what, but it seemed harder than the AM?? Ethics in the PM were hard.

Mechanic

Turn to pg 326 in Volume 1  CFA book.

Mechanic

that is what i memorized too!