# [R-SIG-Finance] A question on portfolio value calculation

Thu Jan 6 13:50:46 CET 2011

```Have a look at this note:
http://papers.ssrn.com/sol3/papers.cfm?abstract_id=1675067
This may help
Dave

On 01/06/2011 01:28 PM, Guy Green wrote:
> In any realistic portfolio you will have some starting equity, and you would
> also have the costs/proceeds of your long & short positions.  The portfolio
> value would then be:
>
> Starting equity + \$(m1-m2-m3) -cost(pos1) +proceeds(pos2) +proceeds(pos2)
>
> or to put it another way:
>
> Starting equity +/- unrealised gains/losses on your positions.
>
> sense that your starting equity is a real-world constraint on the sizes of
> the positions that your broker will allow you to enter into, and also in the
> more theoretical (but still real-world) sense that the relative sizes of
> your starting equity and your positions contribute to the likelihood of your
> strategy exhausting all your equity at some point in the future, even if it
> is a winning one over the long term.
>
> Guy
>
>
> Megh Dal wrote:
>> Hi all, can somebody suggest me on what is the correct way to calculate
>> value of a portfolio (i.e. mark-to-market value) with having both long and
>> short position? For example, suppose I have 3 positions in my portfolio
>> pos1, po2, and pos3 and type of transaction is long, short, short
>> respectively.
>>
>> Say, m2m value of those 3 positions are m1, m2 and m3 in money term. Then
>> should m2m value of this portfolio be \$(m1-m2-m3)?
>>
>> If this is correct I feel there are some practical problem with this
>> approach. Let say I calculated the volatility of this portfolio assuming
>> some normal distribution of return, let say it is \$X. Then if I want to
>> answer, what is the volatility for per unit value of my entire portfolio
>> the
>> answer would be : \$X/\$(m1-m2-m3). However if it happenes that \$(m1-m2-m3)
>> =
>> 0 then above calculation becomes undefined.
>>
>> This approach also may be problametic if I have all short, in this case
>> unit
>> SD for my portfolio becomes obviously negative.
>>
>> Or should I go with \$(abs(m1)+abs(m2)+abs(m3)) to avoid above scenario?
>>
>> Any explanation would be highly appreciated.
>>
>> Thanks
>>

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