Standard Deviation of Sets X & Y: s_{XY}=s_{X}\overline{Y}+s_{Y}\overline{X}?

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Discussion Overview

The discussion revolves around the validity of a proposed formula for calculating the standard deviation of the product of two sets, X and Y. Participants explore the implications of the formula, its conditions, and the definitions of the sets involved.

Discussion Character

  • Debate/contested

Main Points Raised

  • One participant questions the general validity of the formula s_{XY}=s_{X}\overline{Y}+s_{Y}\overline{X}, suggesting it may only hold in specific cases.
  • Another participant seeks clarification on the definitions of sets X and Y, and the meaning of the product XY, noting that the labeling of elements could affect the interpretation.
  • A different participant proposes that the formula might represent the standard deviation of the union of X and Y, but expresses uncertainty about proving this and acknowledges potential contradictions with specific values for means and standard deviations.
  • Concerns are raised about the assumption that the sizes of sets X and Y are equal, which may not always be the case.

Areas of Agreement / Disagreement

Participants express differing views on the validity of the proposed formula, with no consensus reached on its general applicability or the definitions of the sets involved.

Contextual Notes

Participants highlight limitations regarding the assumptions about the distributions of X and Y, the equality of their sizes, and the specific conditions under which the formula might hold.

amcavoy
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For sets X and Y, is it true that

[tex]s_{XY}=s_{X}\overline{Y}+s_{Y}\overline{X},[/tex]​

where [tex]s[/tex] represents the standard deviation and [tex]XY[/tex] is the set containing [tex]x_{i}y_{i}[/tex]?
 
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First off, I assume you mean sample deviation? Standard deviation is a constant.

I doubt the result is true in general. However, there may be a specific case where the formula holds.

Do you have any idea what X and Y might be distributed as? Where does the problem arise?
 
What are X and Y, and what does XY really mean? How can x_iy_i make sense, since this will completely depend on how one labels elements of the sets (apparently they're sets) X and Y? It also presupposes that |X|=|Y| too.
 
Hi there. Remember me? :)

I think it means that the standard deviation of the union of X and Y is equal to the standard deviation of x * the mean of Y + the standard deviation of y * the mean of X.

Uggh I don't know how to prove those - have to go to some statistics textbooks...

Anyways, if Y mean = 100 and Y SD = 0, and X mean = 0 and X SD = 0, then the formula would compute a combined SD of 0. But then your combined sample has both elements of 0 and 100, and it must have a standard deviation. So the formula is not universally true.
 
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