Reporting: weighted average price source
cedayiv at gmail.com
Mon Jul 7 12:42:16 EDT 2008
On Mon, Jul 7, 2008 at 8:53 AM, Derek Atkins <warlord at mit.edu> wrote:
> "David G. Hamblen" <dhamblen at roadrunner.com> writes:
> > A few years back (v1.8x), I had problems with these absolute values, and
> > patched report-utilities.scm,and commodity-utilites.scm so that the
> > sheet would balance. In addition to completely removing all the
> > I also had to do something about the division by zero in
> > 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
> > 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.
> Of course, then you need to make sure you re-apply the negative
> 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.
> 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 PGP key available
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