ars posted a followup article
http://arstechnica.com/gaming/2014/04/steam-gauge-addressing-your-questions-and-concerns/
Since we finally unveiled our Steam Gauge project late last night, we've been overwhelmed by positive responses to the data. It's been come from all over—comment threads, Twitter, e-mail, and links from other sites. It's much appreciated.
We've also received some questions and concerns about our data, our methodology, and what we plan to do with this project going forward. Here are some responses to the most common issues that have been brought up in the last 24 hours or so.
Isn't your data off? Steam didn't always track gameplay hours in the past
Indeed. Before posting our analysis last night, I was not aware that Steam only started tracking the "number of hours played" statistics on SteamCommunity.com in March of 2009. This isn't a small oversight: games played solely before this date would show up erroneously as "unplayed" in our data, and games released before that time might show fewer total hours than they should. This helps explain why older games like Ricochet and Deathmatch Classic seem so unpopular among people who own them—because most players probably put in their hours before March of 2009.
To be clear, this issue should not affect the "ownership" data in the original piece—games bought at any time appear in the scraped Steam data correctly. For some of the other charts, it simply means that games from the pre-2009 period can't be compared completely accurately to those released after March of 2009. I've noted this in the original piece and updated a number of charts to reflect this.
How accurate are your numbers?
Since publishing, we've had a few more developers reach out either privately or publicly to offer their own Steam sales data for comparison to our estimates. With over a dozen "real world" spot checks in hand now, we have yet to see an instance where the error in our numbers is more than 10 percent off from the actual numbers developers have access to. Sometimes our error is much less than that, of course, and the error can go in either direction (though so far our numbers seem to over-estimate slightly more often than under-estimating).
While 10 percent isn't a small functional margin of error, it's also much better than a simple shot in the dark guess. If we're reporting sales of two million units for a game, you can be pretty confident the actual sales number is somewhere between 1.8 and 2.2 million.
http://arstechnica.com/gaming/2014/04/steam-gauge-addressing-your-questions-and-concerns/
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