Re: 95% CI for a sum

[Rich Ulrich]

On 21 Jan 2002 08:55:21 -0800, [EMAIL PROTECTED] (Scheltema,
Karen) wrote:

 I have 2 independent samples and the standard errors and n's associated with
 each of them.  If a and b are constants, what is the formula for the 95%
 confidence interval for 
 (a(Xbar1)+b(xbar2))?
 

Var(A+B) = Var(A) + Var(B)for A,B independent;

and
Var(kA) = k^2  Var(A)

Right?  The SE  is the SD of the mean, 
so you have the terms you need.

-- 
Rich Ulrich, [EMAIL PROTECTED]

http://www.pitt.edu/~wpilib/index.html


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Re: 95% CI for a sum

[Glen Barnett]

Scheltema, Karen wrote:
 
 I have 2 independent samples and the standard errors and n's associated with
 each of them.  If a and b are constants, what is the formula for the 95%
 confidence interval for
 (a(Xbar1)+b(xbar2))?

Are the sample sizes big enough that you'd be prepared to use the CLT?

Glen


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