If a frequency distribution of a population conforms to the so-called
normal distribution, then, what percentage of measurments in the
population fall within two standard deviations of the population mean?

Hi douger-ga,

Assuming a normal distribution, 95% of the measurements will fall
within tow standard deviations of the population mean. The
"68-95-99.7" rule (also called "empirical" rule) answers your question
quite clearly. 68% of the measurements will fall within one standard
deviation, 95% of the measurements will fall within two standard
deviations and 99.7% of the measurements will fall within three
standard deviations.

Here are some sites where you can read more about this rule:

http://www.oswego.edu/~srp/stats/6895997.htm

http://www.socsci.mcmaster.ca/magee/2j3/pages/app2.html

http://www.wayworld.com/samples/SPC/spc_05.htm

Hope this helps!

playhosea-ga

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Clarification of Answer by playhosea-ga on 02 Jul 2003 21:51 PDT

sorry for the typo, the first sentence should read "Assuming a normal
distribution, 95% of the measurements will fall
within two standard deviations of the population mean."