Re: how to determine the variance of dependant variable when all the var. of ind. variables r known in a linear relation

thank everyone of u for helping me here. I personally don't mind the
way of Greg's "teaching". His way might be not proper in this board
(which i am not sure), but he intended to help me out. I think
encouragement of engaging is more important is such public groups,
though good suggestions are always helpful.

Old Mac User wrote:
Greg...

I think it would be nice if you would give this person the answer
without asking him/her to dig further into VAR(y) = E{ (y - E{y} )^2
}

IMHO, when a person comes to this board with a reasonable question...
expresses that question in "plain English"... then we should provide a
direct and practical answer without asking them to "learn all about
statistics".

Tutoring students who are attending courses for credit is one thing.
Helping someone with a simple question is another. As a consultant I'm
expected to "answer questions", not drive my clients to dig into
matters of little or no concern to them.

Full Disclosure: I'm a chemical engineer and also a statistician.

Something like...

Var(y) = (a^2) *Var(X1) + (b^2) * Var(X2)

assuming that a, b, and c are constants which are not subject to random
variation
and assuming that the errors in X1 and X2 are not correlated with each
other.

OMU


Greg Heath wrote:
Jerry wrote:
hi, here is the question,

if we know,
y = a * X1 + b * X2 + c

and we also know the variance of X1 and X2, how to evaluate the
variance of the Y?

Plug in and simplify:

VAR(y) = E{ (y - E{y} )^2 }

Hope this helps.

Greg

.

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