# standard deviation

• Jun 6th 2011, 03:55 AM
percyt
standard deviation

if the standard deviation of X is 15 , what would be the standard deviation of 5X ?

all help is greatly appreciated
• Jun 6th 2011, 04:27 AM
CaptainBlack
Quote:

Originally Posted by percyt

if the standard deviation of X is 15 , what would be the standard deviation of 5X ?

all help is greatly appreciated

How has the standard deviation of a random variable been defined for you?

CB
• Jun 6th 2011, 01:05 PM
mr fantastic
Quote:

Originally Posted by percyt

if the standard deviation of X is 15 , what would be the standard deviation of 5X ?

all help is greatly appreciated

You're expected to know that Var(aX + b) = a^2 Var(X) and therefore sd(aX + b) = |a|sd(X).
• Jun 6th 2011, 01:06 PM
percyt
mr fantastic , thank you ill have a look at that
• Jun 6th 2011, 01:13 PM
mr fantastic
Quote:

Originally Posted by percyt
mr fantastic , thank you ill have a look at that

Before you edited a perfectly good reply to CB to what I have, unfortunately, had to quote (and which, to be honest, is useless compared to what you had originally posted in reply to CB), what you said contained important information. Next time post the whole problem.

From what you said before deleting it, the sd deviation you're talking about cannot be calculated without knowing the values of X. Then apply the usual formula to the data. Formulas can be found using Google.