[R-SIG-Finance] Value-at-risk

sadako guillaume.kovarcik at gmail.com
Sun Jun 19 19:20:30 CEST 2011

sadako wrote:
>> you can see the code with: PerformanceAnalytics:::VaR.Marginal
> I'm having a look, maybe the difference stems from the application of
> Return.portfolio in the marginal case...

I think we don't get the same univariate portfolio VaR with the two
portfolio_method "marginal" and "component" because of :

- in PerformanceAnalytics:::VaR.Marginal, the Return.portfolio are
calculated without the optional argument geometric (geometric=FALSE would
eventually match the stdev I compute).

- in PerformanceAnalytics:::VaR.Marginal, when calling the
portfolio_method="single" to compute the univariate portfolio VaR, we end up
in the PerformanceAnalytics:::VaR.Gaussian function. 
This function uses the PerformanceAnalytics:::centeredmoment function, which
uses the mean function. 
This does not give the same variance as stdev for instance since there's not
the ajustement of the estimator (division by n-1 instead of n if data set
has n observations). 
If we set m2 = centeredmoment(r, 2)*dim(r)[1]/(dim(r)[1]-1), it looks ok.

With these two modifications, I have the impression the univariate portfolio
VaR computed from portfolio_method="marginal" and
portfolio_method="component" are consistant.

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