standard deviation (x / 12)

[jimjohn]

if i know what the standard deviation of x is, can someone plz tell me what the formula for standard deviation of x / 12 is?

it seems that if standard deviatino of x = y, then standard deviatino of x / 12
 = y * SQRT (1/12)
can someone plz confirm if this is right, and if so how do we get this formula. i cant figure it out intuitively. thx.

[Richard Ristow]

At 10:30 AM 3/31/2008, jimjohn wrote:

>if i know what the standard deviation of x is, can someone plz tell
>me what the formula for standard deviation of x / 12 is?
>
>it seems that if standard deviatino of x = y, then standard deviation
>of x /12  = y * SQRT (1/12)

No. Standard deviation scales linearly. (It pretty well has to, or
you could change the ratio of means to standard deviations, and hence
significance values in many tests, by multiplying by a constant.)

If y=x/12

StdDev(y) = StdDev(x)/12

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