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

Mechanic

that is what i memorized too!

Mechanic

Turn to pg 326 in Volume 1  CFA book.

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.

dece2011

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

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.

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%

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.

btcapital

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