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GW Auctions Forums - View topic - median?
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median? 
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Joined: Sun Nov 02, 2008 9:16 pm
Posts: 40
Post median?
Could we also get a "median" value in the price records display? "Average" can be skewed by 1 outrageously over or under priced item.


Thu Feb 19, 2009 3:54 pm
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Post Re: median?
Not a bad idea. I remember vaguely learning in high school that there are 3 different averages and depending on the situation, all three are valid. Which one is most useful depends on the situation.

Mean, Mode or Median.

Mean - this is the most common and what is usually meant by "average". If an item has been sold 10 times then add up all the prices and divide by 10. This is the mean or average price of an item. Like Urass said, it can be skewed by 1 item that's way off base. Look at the prices for Celestial Shields. One of these shields sold for 500k. :o That's not even feasible and I suspect it was a fake auction and the trade never took place. That sale though has skewed the price for Celestial shields way up.

Mode - Mode is simply to look at all data and select the value that occurs most. Look at the price records for the Strength and Honor inscription. 32 have been sold at various prices. However most of them were sold for 1k. This is the mode price, or most common price paid. The average (mean) price is 991 so that's close.

Median - This is the one that Uras wants added. It's the price that's right smack dab in the middle of the range. If you have 11 items sold then you sort them by price and look at the price item #6 cost. If you have a lot of data then this is usually accurate. For the Strength and Honor inscriptions there were 32 sold and #16 and #17 were both 1k.

If feasible, add all 3 averages. The programming should be simple, it's just a matter of how high a priority it should be. I'm sure there are other more pressing issues.


Thu Feb 19, 2009 6:00 pm
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Post Re: median?
There is a way of doing this so that you capture the range of prices and then discount the top and bottom prices and then sample the median range from where you would select your new top and bottom start- and end-of-range.

It is what they do for "six sigma" sampling so that the skew, to which Urass refers, is eliminated. The only problem is that the formula is quite complicated and has to be revised for each sampling exercise.

As it stands now the list of items in the Price Records does show the range and I am sure that discerning individuals probably do their own visual sampling and are able to ignore extremely high or low bids for the item and see their own avaerage price from amongst the trees that make up the lists.

Bearz


Thu Feb 19, 2009 8:02 pm
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Post Re: median?
I'm sure there is a "median" function that would require 1 line of code to add :)


Thu Feb 19, 2009 8:57 pm
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Joined: Thu Apr 17, 2008 6:56 pm
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Post Re: median?
This might be useful.
I dont think I would use it, because if there are 10 items sold, I just look at what is a good price comparing all 10 items.
If there are 100 items sold, the average(mean) comes close enough probably.

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Thu Feb 19, 2009 10:33 pm
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Post Re: median?
well... The mean can be calculated "as you go". You don't need all values to start calculating it. The median requires you to know how many values you have in total and then select the middle one. This wouldn't be hard except that one auction could represent 5 of the same items.

With the mean, you can use a regular expression to determine the amount of items auctioned and calculate the new mean right there and then. With the median, you would first have to loop all records to find the number of items auctioned and construct a new array which can then be used for the calculation. An auction of 5 Globs of Ectoplasm would have to add 5 entries to the new array. An auction of 250 Piles of Glittering Dust would have to add 250 entries. It's possible but quite a big increase in memory usage, especially when there are no search parameters given. Not to mention that the new array also has to be sorted on price while the list you requested may have been sorted on date. And sorting such a large array is quite CPU intensive.

Instead of adding 250 entries for 250 piles you could also have a two-dimensional array containing single price and number of occurrences. Less memory intensive and can also be used to get the mode. It would still has to be sorted and a drawback is that it has to be looped (this would be the second loop) after construction to find the median.

The big question is, is it worth it? Of all the three averages, the mean is definitely the cheapest to calculate. The mode is very unreliable here and not worth it. It would consider 1k4g and 1k5g different values. The median is more interesting but a lot more costly than the mean.


Fri Feb 20, 2009 9:50 pm
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Post Re: median?


Fri Feb 20, 2009 10:45 pm
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Post Re: median?


Fri Feb 20, 2009 11:11 pm
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Post Re: median?
There is something else to consider when looking at Price Records functionality and Effort vs Value to improve it.

"The other place" to my mind epitomises the nature of the GW Economy in-game where there are as many Buyers as Sellers and prices and price checks reflect a desire to almost see Sellers out-do one another in seeing how much they can get for the same item and how they 'price' items for people asking for "Price Checks".

GW Auctions has a surfeit of Sellers and still too few Buyers leading to prices in GW Auctions being depressed due to the simple economics of Supply and Demand. This has been so for a long while.

Thus, to my mind, the Price Records are only a guide to GW Auction prices and not a reflection of universal Guild Wars market value as the Price Records are already skewed in favour of lower prices in GW Auctions than might be achieved in-game.

Until the number of Buyers operating within the GW Auctions environment increases, I think Price Records will remain intrinsically skewed towards almost too low prices and thus any efforts to remove the skew of unrealistic too low or too high prices will fail to eliminate the pre-existing skew caused by having too many Sellers versus too few Buyers in GW Auctions! ;)

I am not complaining though as I have made a fortune from buying in GW Auctions but it is something to consider. :)

Bearz


Sat Feb 21, 2009 10:06 am
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