ok i got the mean which is 300 and the SD is 70. What is the first quartile score? how can i get this number?

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- Sep 18th 2008, 08:54 AMmakaveli89SD and Mean
ok i got the mean which is 300 and the SD is 70. What is the first quartile score? how can i get this number?

- Sep 18th 2008, 01:18 PMCaptainBlack
- Sep 18th 2008, 07:43 PMmakaveli89
i dont get it

- Sep 18th 2008, 08:54 PMCaptainBlack
If you have calculated the mean and standard deviation from a sample then you need to calculate the 1-st quartile from the sample, you can't get it from the maen and standard deviation.

If the mean and standard deviation are from a theoretical distribution you need to know what that distribution is to calculate the 1-st quartile.

In other words post the entire question.

RonL - Sep 19th 2008, 08:24 AMmakaveli89
it is normally distributed with a mean of 300 and a standard deviation of 70.

- Sep 19th 2008, 09:15 AMCaptainBlack
Let $\displaystyle Q$ denote the cumulative distribution of the standard normal distribution. The the 1-st Quartile of the standard normal is:

$\displaystyle q_1=Q(0.25)$

and that of a normal distribution of mean $\displaystyle \mu$ and sd $\displaystyle \sigma$ is:

$\displaystyle q_1(\mu,\sigma) = \mu + \sigma q_1$

RonL - Sep 19th 2008, 01:45 PMmr fantastic