Standard deviations, etc.

1. Nov 14, 2007

Sir

in my analytical chemistry review, this question cropped up. the first part was simple, but the second part has me confused to no end. i've tried playing around with the math as well as scouring my textbook for answers. help!

what is the probability that a single determination of a value is further below the mean than 2 standard deviations?

this part is the simpler part, given the table at the bottom of this page.
the answer turns out to be 2.28%

the second part of the problem is: what is the probability that the average of 4 determinations is further below the mean than 2 standard deviations?

the answer was given to be 0.0032% but i cannot figure out where this value comes from.

ps. my apologies for not reading the sticky, I'll post this in the proper forums now.

Last edited: Nov 14, 2007
2. Nov 16, 2007