Reporting: weighted average price source
Charles Day
cedayiv at gmail.com
Tue Jul 8 13:29:18 EDT 2008
On Tue, Jul 8, 2008 at 8:50 AM, Charles Day <cedayiv at gmail.com> wrote:
> On Tue, Jul 8, 2008 at 5:59 AM, David G. Hamblen <dhamblen at roadrunner.com>
> wrote:
>
>> Charles Day wrote:
>>
>>> On Mon, Jul 7, 2008 at 8:53 AM, Derek Atkins <warlord at mit.edu <mailto:
>>> warlord at mit.edu>> wrote:
>>>
>>> "David G. Hamblen" <dhamblen at roadrunner.com
>>> <mailto:dhamblen at roadrunner.com>> writes:
>>>
>>> > A few years back (v1.8x), I had problems with these absolute
>>> values, and I
>>> > patched report-utilities.scm,and commodity-utilites.scm so that
>>> the balance
>>> > sheet would balance. In addition to completely removing all the
>>> numeric:abs,
>>> > I also had to do something about the division by zero in
>>> commodity-utilities
>>> > when there was a zero share balance. I'm using the "Nearest in
>>> Time" and the
>>> > problem went away in the 2.x updates. If anyone's interested, I
>>> can dig up
>>> > my old postings.
>>> >
>>> > Anyway, I vote for getting rid of absolute values in bookkeeping.
>>>
>>> It's not a question of book keeping. It's a question of computing
>>> the share price. GnuCash stores buys as a positive and sells as
>>> a negative. As Christian pointed out before, if you buy x shares
>>> for $y and then later SELL x shares for $y then a non-abs weighted
>>> average gives you a price of 0/share! Obviously this is wrong.
>>> The weighted average should be $(y/x) per share. But without
>>> the ABS you get:
>>>
>>> y * x + (-y)*x xy -xy 0
>>> -------------- == ------ == --- = 0
>>> x + x 2x 2x
>>>
>>> When you use abs() here you get the right answer.
>>>
>>> The fundamental issue here is that I don't think that the average price
>> should include shares I no longer own.
>>
>> My tests (all using the Balance Sheet) indicate that share prices (the
>> y's) are all positive and are calculated from each transaction (Total
>> Buy/Total Shares). The Total Buy (the x*y product) and the Total shares (the
>> x's) are both positive or both negative. What seemed to be happening back
>> in 1.8x, and is still happening in 2.4) was that when you have zero shares,
>> the answer for the weighted average is irrevelant, but I have a situation
>> where I sold all my shares (as in your example), but subsequently purchased
>> a few more shares of the same stock. The weighted average (without all the
>> abs()'s) looks like this: (I can't wait to see what Thunderbird does with my
>> ascii equations :-) )
>>
>> y * x + y * (-x) + z * w z * w
>> ----------------------------------- == --------- == w
>> x + (-x) + z z
>>
>> With the abs(), I get a nonsensical average price for my holdings in this
>> asset. In my specific test case, with the initial purchase/sell of 100
>> shares at $5 (as in your example), and a subsequent purchase of 10 shares at
>> $20, I get a weighted average price of $5.714, and an asset worth $57.14.
>> Using the "Nearest in Time" or the "Most Recent" price source, I get an
>> average share price of $20, and an asset value of $200.
>> Without the abs(), you get 0/0 for the weighted average in your example,
>> so that needs to be trapped.
>>
>
> So far as I can tell, this formula is the same as the new price source
> named "Average Cost" that I added yesterday (r17266).
>
>
>>
>> The good news is that in 2.4, the Balance Sheet balances in all cases (not
>> true in 1.8x).
>>
>
> Unless you have unrealized gains or losses on liabilities. I'm working on
> that...
>
...done. As of r17287, the balance sheet report now supports calculation of
unrealized gains and losses on liabilities.
> -Charles
>
> for sales.
>>>
>>> So it's not a question of absolute values in bookkeeping. It's
>>> a question of absolute values in computing share prices. Not
>>> the same thing.
>>>
>>>
>>> I agree with Derek. That is, the "weighted average" price source is not
>>> intended to compute the cost of your holdings, but rather to compute the
>>> volume-weighted average price of all buys AND sells. So it needs to keep
>>> using the absolute value. However, it does need to be changed: it should
>>> ignore exchanges with a zero "amount" in the split.
>>>
>>> So I think we need to add a new price source of "Cost" which computes
>>> without absolute value and does include zero "amount" splits. That's where
>>> taking a look at David's code might come in handy (though implementing this
>>> seems like a pretty simple job: copy existing weighted average function, get
>>> rid of absolute value, add report option).
>>>
>>> Whether to even keep the "weighted average" price source is a question
>>> worth asking; I don't think it is a useful way to revalue current holdings.
>>> Historical volumes don't seem relevant. Even if the volume weighting was
>>> removed, the formula also weights quotes that are clustered within a small
>>> period of time more heavily than quotes that are spread apart. In other
>>> words, if I have 30 quotes from this month and one quote from last year, the
>>> recent quotes practically drown out the quote from last year. If we want to
>>> aim for a historical price average then using something like linear
>>> regression seems like a better way to go. Could even add some time factors,
>>> like getting the midpoint of the portion of the line covering the last 30,
>>> 60, 90 days, whatever. But now I am brainstorming new price sources.
>>>
>>> I do think a volume-weighted average price can be useful; it's just more
>>> appropriate for measuring your personal trading performance. Maybe there
>>> should be a performance report designed around that.
>>>
>>> -Charles
>>>
>>> derek
>>> --
>>> Derek Atkins, SB '93 MIT EE, SM '95 MIT Media Laboratory
>>> Member, MIT Student Information Processing Board (SIPB)
>>> URL: http://web.mit.edu/warlord/ PP-ASEL-IA N1NWH
>>> warlord at MIT.EDU <mailto:warlord at MIT.EDU>
>>> PGP key available
>>>
>>>
>>> Of course, then you need to make sure you re-apply the negative
>>
>>
>>
>
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