  
| Author: | FigBits  | | Posted: | Jul 20, 2016 12:41 | | Subject: | Re: standard deviation on price indices | | Viewed: | 39 times | | Topic: | Suggestions | |
|
| In Suggestions, bljesprop writes:
| | In Suggestions, FigBits writes:
| | In Suggestions, bljesprop writes:
| | In Suggestions, FigBits writes:
| | In Suggestions, brick_projects writes:
| | Please implement standard deviation.
By eliminating the (small number)% cheapest and (small number) % most expensively
priced lots, the average price index would be "more statistically significant"
in scientific terms ( less error margin ).
This is only necessary for items that have at least e.g. 10 distinct lots for
sale.
Why this suggestion ?
Yesterday i noticed one particular seller set all his prices at ( BL avg x 10000
). This caused average price index "lots for sale" to be inflated at least
tenfold.
Someone else suggested he made a mistake in currency.
If this one shopkeeper persists, unwanted "extreme" inflation may occur on several
very common items, and those lots - for sellers not paying attention when adding
to their inventory - might become impossible to sell.
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Voted no.
I do not want BL to remove any data. If you want to suggest *adding* information,
I would be fine with that. (Such as adding the median price.)
To any seller who blindly uses the current For Sale Average to set their prices,
I would have a simple suggestion: Don't.
So, for the suggestion: No. Please do not remove information which is useful
to many of us, just because a few sellers might be using that information in
a silly way (that only hurts themselves).
--
Marc.
|
Back when I took stats, trimming extreme outliers was considered a desirable
practice. This would be particularly true if the outlier was not so much a black
swan as a data recording error or false reporting. Does anyone know if this
advice has changed?
In any case, the notion of "removing" data does not seem to fully apply here.
No one is suggesting it be removed from the price guide. Someone interested
in reviewing every data point is free to look at them where they are reported
in the price guide.
|
The information that is being suggested to be removed is the mathematical mean.
I don't want that removed. If people want to have additional information
added, such as a trimmed mean, that's fine. But not as a replacement
of what we have now.
| | Frankly, the only reason I can think of for not making this change is that some
sellers will make fewer and smaller mistakes. That will prevent the rest of
us from enjoying the arbitrage.
|
Not all prices have normal distribution. Sometimes the outliers are significant.
http://www.bricklink.com/catalogPG.asp?P=62360&colorID=11
For the last several months, the trimmed mean for this part would have shown
around $1. The actual mean (which is what currently gets displayed) was $2. If
I had some last month, I probably would have listed them at $15, which would
likely have been the highest listing at the time. And they probably would have
sold by now.
Now, in that case, the mean isn't a particularly good representation either.
But it's a better number for a seller wondering what price they should ask,
than the trimmed mean.
--
Marc.
|
As you note, your example really does not prove that one or the other is superior.
However, a measurement of the price trend might have helped to identify the
impact of the part being used in a MOC. Better still, knowing that the demand
for the part was about to change would have tipped a seller not to use the average
price heuristic. T
In the parts of the catalog I typically study, a trimmed mean would be a much
better choice as the first option. It certainly should be easy to provide both.
The problem being addressed how sellers can quickly price a large variety of
inexpensive items without getting it wrong. Price trends would enhance this
information. A warning that the amount of transactions is low (ie that attributes
other than price may have greater impact) would also be helpful (and would have
also flagged your example).
|
I agree with all that.
As always, more information is better than less information.
--
Marc.
|
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Message is in Reply To: 21 Messages in this Thread:  Msg 1 - brick_projects (3653) 96 months ago Jul 20, 2016 to Suggestions
 Msg 2 - FigBits (3563) 96 months ago Jul 20, 2016 to Suggestions
  Msg 3 - jeslego (1050) 96 months ago Jul 20, 2016 to Suggestions
Msg 4 - FigBits (3563) 96 months ago Jul 20, 2016 to Suggestions
Msg 5 - yorbrick (1185) 96 months ago Jul 20, 2016 to Suggestions
Msg 6 - FigBits (3563) 96 months ago Jul 20, 2016 to Suggestions
Msg 7 - jeslego (1050) 96 months ago Jul 20, 2016 to Suggestions
Msg 8 « - FigBits (3563) 96 months ago Jul 20, 2016 to Suggestions
Msg 9 - jedvii (666) 96 months ago Jul 22, 2016 to Suggestions
Msg 10 - yorbrick (1185) 96 months ago Jul 20, 2016 to Suggestions
Msg 11 - jancg (378) 96 months ago Jul 20, 2016 to Suggestions
Msg 12 - ToddMyers (356) 96 months ago Jul 20, 2016 to Suggestions
Msg 13 - jancg (378) 96 months ago Jul 20, 2016 to Suggestions
Msg 14 - Leftoverbricks (2225) 96 months ago Jul 20, 2016 to Suggestions
Msg 15 - garbanzo (322) 96 months ago Jul 20, 2016 to Suggestions
Msg 16 - jancg (378) 96 months ago Jul 20, 2016 to Suggestions
Msg 17 - yorbrick (1185) 96 months ago Jul 20, 2016 to Suggestions
Msg 18 - ToriHada (8887) 96 months ago Jul 20, 2016 to Suggestions
Msg 19 - Made_In_Bricks (3994) 96 months ago Jul 20, 2016 to Suggestions
Msg 20 - FigBits (3563) 96 months ago Jul 20, 2016 to Suggestions
Msg 21 - wildchicken13 (876) 43 months ago Nov 17, 2020 to Suggestions
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Canada, Ontario
