[R] Calculating Portfolio Standard deviation

[Patrick Burns]

That should be the variance matrix
of returns, not prices.  (I have a
blog post on this already written
that will be published later this week.)

On 10/01/2011 09:05, Joshua Wiley wrote:
> Dear Amelia,
>
> If you have the actual data you should be able to use the variance covariance matrix to simplify this
>
> Vdat<- cov(prices_df)
>
> sum(diag(Vdat)) + 2*Vdat[upper.tri(Vdat)]
>
> By using covariances instead of correlations you do not need to multiply by he standard deviations and by using variances there's no need to square. The only trick would be adding your weights back in.  See ?diag and ?upper.tri and ?vcov for relevant documentation.
>
> Cheers,
>
> Josh
>
> On Jan 10, 2011, at 0:26, Amelia Vettori<amelia_vettori at yahoo.co.nz>  wrote:
>
>> Dear R helpers
>>
>> I have following data
>>
>> stocks<- c("ABC", "DEF", "GHI", "JKL")
>>
>> prices_df<- data.frame(ABC = c(17,24,15,22,16,22,17,22,15,19),
>>                                           DEF = c(22,28,20,20,28,26,29,18,24,21),
>>                                            GHI = c(32,27,32,36,37,37,34,23,25,32),
>>
>>                                            JKL = c(47,60,60,43,62,38,44,53,61,41))
>>
>> sd_prices<- c(3.3483,3.9497,4.9721,9.3387)   # standard deviations say(sd1, sd2, sd3, sd4)
>>
>> weights<- c(0.10, 0.25, 0.20, 0.45)      # say (w1, w2, w3, w4)
>>
>> I need to calculate the standard deviation of the portfolio. The formula is
>>
>> stdev_portfolio = sqrt((w1*sd1)^2+(w2*sd2)^2+(w3*sd3)^2+(w4*sd4)^2 +
>>                          2*w1*w2*sd1*sd2*correlation(ABC, DEF)+
>>
>>                       2*w1*w3*sd1*sd3*correlation(ABC, GHI)+
>>                          2*w1*w4*sd1*sd4*correlation(ABC, JKL)+
>>                          2*w2*w3*sd2*sd3*correlation(DEF, GHI)+
>>                          2*w2*w4*sd2*sd4*correlation(DEF, JKL)+
>>                          2*w3*w4*sd3*sd4*correlation(GHI, JKL))
>>
>>
>>
>> OR if we define
>>
>> P = sd_prices*weights
>>
>> I need to calculate
>>
>> stdev_portfolio = sqrt((P1)^2+(P2)^2+(P3)^2+(P4)^2 +
>>                          2*P1*P2*correlation(ABC, DEF)+
>>                          2*P1*P3*correlation(ABC, GHI)+
>>                          2*P1*P4*correlation(ABC, JKL)+
>>                          2*P2*P3*correlation(DEF,
>> GHI)+
>>                          2*P2*P4*correlation(DEF, JKL)+
>>                          2*P3*P4*correlation(GHI, JKL))
>>
>> In reality I will be dealing with not 4, but many stocks and hence I can't generalize this as
>>
>> stdev_portfolio = sqrt((P[1])^2+(P[2])^2+(P[3])^2+(P[4)^2 +
>>                          2*P[1]*P[2]*correlation(ABC, DEF)+
>>                          2*P1*P3*correlation(ABC,
>> GHI)+
>>                          2*P1*P4*correlation(ABC, JKL)+
>>                          2*P2*P3*correlation(DEF, GHI)+
>>                          2*P2*P4*correlation(DEF, JKL)+
>>                          2*P3*P4*correlation(GHI, JKL))
>>
>> Kindly advise as to how do I
>> calculate the portfolio standard deviation?
>>
>> Thanking in advance
>>
>> Amelia Vettori
>>
>>
>>
>>     [[alternative HTML version deleted]]
>>
>> ______________________________________________
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>> https://stat.ethz.ch/mailman/listinfo/r-help
>> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
>> and provide commented, minimal, self-contained, reproducible code.
>
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>

-- 
Patrick Burns
pburns at pburns.seanet.com
twitter: @portfolioprobe
http://www.portfolioprobe.com/blog
http://www.burns-stat.com
(home of 'Some hints for the R beginner'
and 'The R Inferno')