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standard deviation

country a annual exports to country b are $3.2bilion on average with standard deviation of 0.28 billion. country a annual imports to country b are $3.5 bilion on average with standard deviation of 0.24 billion. Calculate the standard deviation of country a balance of trade with country b (that is exports - imports). 

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DEL R. | Expert Math and Stats TutorExpert Math and Stats Tutor
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assuming independence of imports and exports, calculate the variance first
 
var (Export - Import) = var (export) + Var (Import)    [ yes, +, not -]
                               = 0.28^2 + 0.24 ^2
 
Then, SD (Export - Import)  = sqrt [ var (Export - Import) ]