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From:  Date: Tue, 12 Jan 1999 15:06:50 +0800

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 Hi statistical net members : I am not in the field of statistics but I joined this group to get some of my basic doubts clarified.I work as a manufacturing engineer and I use statistical methods for some of my improvement projects.One of these projects is the tolerance analysis using statistics.I do tolerance analyses of my design to provide/predict tolerances required / to be provided by vendors etc. Question 1) If I have a variable X which is a function of three other variables A,B,C and is related by this relation : X=A+B+C Can I apply sum of square rule on the variances and say , Variance (X) = Variance (A) + Variance (B) + Variance (C) That is (STD. DEV X)^2 = (STD. DEV A)^2 + (STD. DEV B)^2 + (STD. DEV C)^2 Am I correct ? Question 2) If I have a variable X which is a function of three other variables A,B,C and is related by this relation : X=A+B+(C/2) Can I apply sum of square rule on the variances and say , (STD. DEV X)^2 = (STD. DEV A)^2 + (STD. DEV B)^2 + (STD. DEV (C/2) )^2 OR (STD. DEV X)^2 = (STD. DEV A)^2 + (STD. DEV B)^2 + ( (STD. DEV (C) )^2 divided by 4 ) Which is correct ? Question 3) If I have a variable X which is a function of three other variables A,B,C and is related by this relation : X=A-B+C Can I apply sum of square rule on the variances and say , (STD. DEV X)^2 = (STD. DEV A)^2 + (STD. DEV B)^2 + (STD. DEV C)^2 OR (STD. DEV X)^2 = (STD. DEV A)^2 - (STD. DEV B)^2 + (STD. DEV C)^2 Which is correct ? Before you answer,please note that I am not a statistical expert and therefore you may need to explain in easier terms !!! .I am not really looking for equations and stuff like that,but I am interested in knowing the correct usage of the SUMOF SQUARES rule. Thanks in advance, Vishy. %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%