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st: Standard error of a sum

From   "Meloria Meschi" <[email protected]>
To   <[email protected]>
Subject   st: Standard error of a sum
Date   Tue, 13 Jun 2006 19:55:11 +0200


I have a silly problem that's driving me nuts.  I have estimated two production function, with AR(1) correction.  I got 7 point forecasts each and their standard errors (call each forecast series x1 and x2).  I have computed the confidence intervals for each of these forecasts. 

Then I have added the two forecasts into one series y=x1 + x2 and produced s.e. and confidence intervals for this series as well.  What I need now is a confidence interval for the sum of y (ie total production from the two sites).  How do I compute the standard error for this total? And does it actually make sense to do so?



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