# SD and Mean

• September 18th 2008, 08:54 AM
makaveli89
SD 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?
• September 18th 2008, 01:18 PM
CaptainBlack
Quote:

Originally Posted by makaveli89
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?

You can't get the quartiles from this information, at least not without additional assumptions about the distribution.

The first quartile $q_1$ is such that:

$p(x < q_1)=1/4$

RonL
• September 18th 2008, 07:43 PM
makaveli89
i dont get it
• September 18th 2008, 08:54 PM
CaptainBlack
Quote:

Originally Posted by makaveli89
i dont get it

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
• September 19th 2008, 08:24 AM
makaveli89
it is normally distributed with a mean of 300 and a standard deviation of 70.
• September 19th 2008, 09:15 AM
CaptainBlack
Quote:

Originally Posted by makaveli89
it is normally distributed with a mean of 300 and a standard deviation of 70.

Let $Q$ denote the cumulative distribution of the standard normal distribution. The the 1-st Quartile of the standard normal is:

$q_1=Q(0.25)$

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

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

RonL
• September 19th 2008, 01:45 PM
mr fantastic
Quote:

Originally Posted by makaveli89
it is normally distributed with a mean of 300 and a standard deviation of 70.

It's funny what a difference two additional words can make. Next time make sure all the information is included.