标题: Negative Standard Deviation? [打印本页] 作者: suyash1989 时间: 2011-7-26 11:13 标题: Negative Standard Deviation?
Ok heres a question i ws thinking about when doing some work in excel...
The variance of a 2 asset port = (Wa^2)(Var a) + (Wb^2)(Var b) + 2(Wa)(Wb)(Std a)(Std b)(Correl (a,b))
My question is this...lets say the correlation is -1. Wouldnt it be possible to get a variance of the portfolio that is negative? What would that mean? or am i thinking a bout this wrong?作者: random_walker47 时间: 2011-7-26 11:19
Variance is squared, removes the negative, and standard dev is obviously the square root of variance which makes it impossible to have a negative standard deviation.
Standard deviation can be zero, but never negative. Both sides of the distribution result in positive value for standard deviation.
I think you're thinking about this one a little too hard.
Edited 2 time(s). Last edit at Thursday, June 23, 2011 at 11:39AM by Chuckrox8.作者: bcp901 时间: 2011-7-26 11:24
Spanishesk Wrote:
-------------------------------------------------------
> Ok heres a question i ws thinking about when doing
> some work in excel...
>
> The variance of a 2 asset port = (Wa^2)(Var a) +
> (Wb^2)(Var b) + 2(Wa)(Wb)(Std a)(Std b)(Correl
> (a,b))
>
> My question is this...lets say the correlation is
> -1. Wouldnt it be possible to get a variance of
> the portfolio that is negative? What would that
> mean? or am i thinking a bout this wrong?
Yes. If the third part of the equation is negative enough to make the summation negative, variance become negative and standard deviation become meaning less as there is no square root for negatives.
However, it is a mathematical truth that third part of the equation can't make the summation negative. The most it can do is making the variance zero (I don't know how to prove it, but know as a fact).作者: noel 时间: 2011-7-26 11:30
The least you can get is when the variances are equal, the weight is equal and correlation is -1. In this case the formula reduces [by letting Wa = Wb, and var(a) = var(b)] to:
2*(Wa^2)(Var(a)) + (-1)*2(Wa^2)(Var(a)) = 0
You will never get a negative variance.作者: tango_gs 时间: 2011-7-26 11:35
Spanishesk Wrote:
-------------------------------------------------------
> Ok heres a question i ws thinking about when doing
> some work in excel...
>
> The variance of a 2 asset port = (Wa^2)(Var a) +
> (Wb^2)(Var b) + 2(Wa)(Wb)(Std a)(Std b)(Correl
> (a,b))
>
> My question is this...lets say the correlation is
> -1. Wouldnt it be possible to get a variance of
> the portfolio that is negative? What would that
> mean? or am i thinking a bout this wrong?
The variance of a 2 asset port = (Wa^2)(Var a) +(Wb^2)(Var b)
+2(Wa)(Wb)(Std a)(Std b)(Correl(a,b)) = (wa*Std a-wb*Std b)^2+2*Wa*Wb*Std a*Std b*(1+cor(a,b)). Each term is non-negative. Therefore, the sum is non-negative.作者: economicz 时间: 2011-7-26 11:40
Forget about formulas. Just think for a second...what is standard deviation? And how can it be negative?作者: NakedPuts2011 时间: 2011-7-26 11:46
Iginla2010 Wrote:
-------------------------------------------------------
> Forget about formulas. Just think for a
> second...what is standard deviation? And how can
> it be negative?
my thoughts exactly....作者: yodacaia 时间: 2011-7-26 11:51
obviously it cant be negative. But mathematically i looked at the equation and started thinking if the third part of the equation was negative enough, it could lead to a negative result. Was just trying to see if it was possible in the equation. I am fully aware that negative std dev wouldnt make sense.作者: sabre 时间: 2011-7-26 11:57
In a two asset portfolio assuming perfect negative correlation your standard deviation would be zero. The third part of the variance equation would most certainly be negative, but the first two expressions would be equal to that amount.
Part of the equation can certainly be less than zero, but it will always be offset by the first two expressions.作者: SeanWest 时间: 2011-7-26 12:07
You guys need to seriously get a life. Evidently I do to for even reading this whole thread...
