Wednesday, December 31, 2014

The Year Amazon Failed Calculus

In August, Amazon sent me a remarkable email containing a treatise on ebook pricing. I quote from it:
... e-books are highly price elastic. This means that when the price goes down, customers buy much more. We've quantified the price elasticity of e-books from repeated measurements across many titles. For every copy an e-book would sell at $14.99, it would sell 1.74 copies if priced at $9.99. So, for example, if customers would buy 100,000 copies of a particular e-book at $14.99, then customers would buy 174,000 copies of that same e-book at $9.99. Total revenue at $14.99 would be $1,499,000. Total revenue at $9.99 is $1,738,000. The important thing to note here is that the lower price is good for all parties involved: the customer is paying 33% less and the author is getting a royalty check 16% larger and being read by an audience that’s 74% larger. The pie is simply bigger.
As you probably know, I'm an engineer, so when I read that paragraph, my reaction was not to write an angry letter to Hachette or to Amazon - my reaction was to start a graph. And I have a third data point to add to the graph. At, I've been working on a rather different price point, $0.  Our "sales" rate is currently about 100,000 copies per year. Our total "sales" revenue for all these books adds up to zero dollars and zero cents. It's even less if you convert to bitcoin.

($0 is terrible for sales revenue, but it's a great price for ebooks that want to accomplish something other than generate sales revenue. Some books want more than anything to make the world a better place, and $0 can help them do that, which is why is trying so hard to support free ebooks.)

So here's my graph of the revenue curve combining "repeated and careful measurements" from Amazon and

I've added a fit to the simplest sensible algebraic equation possible that fits the data, Ax/(1+Bx2), which suggests that the optimum price point is $8.25. Below $8.25,  the increase in unit sales won't make up for the drop in price, and even if the price drops to zero, only twice as many books are sold as at $8.25 - the market for the book saturates.

But Amazon seems to have quit calculus after the first semester, because the real problem has a lot more variables that the one Amazon has solved for. This is because they've ignored the effect of changing a book's price on sales of ALL THE OTHER BOOKS. For you math nerds out there, Amazon has measured a partial derivative when the quantity of interest is the total derivative of revenue. Sales are higher at $10 than at $15 mostly because consumers perceive $15 as expensive for an ebook when most other ebooks are $10. So maybe your pie is bigger, but everyone else is stuck with pop-tarts.

While any individual publisher will find it advantageous to price their books slightly below the prevailing price, the advantage will go away when every publisher lowers its price.

Some price-sensitive readers will read more ebooks if the price is lowered. These are the readers who spend the most on ebooks and are the same readers who patronize libraries. Amazon wants the loyalty of customers so much that they introduced the Kindle Unlimited service. Yes, Amazon is trying to help its customers spend less on their reading obsessions. And yes, Amazon is doing their best to win these customers away from those awful libraries.

But I'm pretty sure that Jeff Bezos passed calculus. He was an EECS major at Princeton (I was, too). So maybe the calculation he's doing is a different one. Maybe his price optimization for ebooks is not maximizing publisher revenue, but rather Amazon's TOTAL revenue. Supposing someone spends less to feed their book habit, doesn't that mean they'll just spend it on something else? And where are they going to spend it? Maybe the best price for Amazon is the price that keeps the customer glued to their Kindle tablet, away from the library and away from the bookstore. The best price for Amazon might be a terrible price for a publisher that wants to sell books.

Read Shatzkin on Publishers vs. Amazon. Then read Hoffelder on the impact of Kindle Unlimited. The last Amazon article you should read this year is Benedict Evans on profits.

It's too late to buy Champagne on Amazon - this New Year's at least.

Sunday, December 7, 2014

Stop Making Web Surveillance Bugs by Mistake!

Since I've been writing about library websites that leak privacy, I figured it would be a good idea to do an audit of to make sure it wasn't leaking privacy in ways I wasn't aware of. I knew that some pages leak some privacy via referer headers to Google, to Twitter, and to Facebook, but we force HTTPS and make sure that user accounts can be pseudonyms. We try not to use any services that push ids for advertising networks. (Facebook "Like" button, I'm looking at you!)

I've worried about using static assets loaded from third party sites. For example, we load jQuery from (it's likely to be cached, and should load faster) and Font Awesome from (ditto). I've verified that these services don't set any cookies and allow caching, which makes it unlikely that they could be used for surveillance of users.

It turned out that my worst privacy leakage was to Creative Commons! I'd been using the button images for the various licenses served from I was surprised to see that id cookies were being sent in the request for these images.
In theory, the folks at Creative Commons could track the usage for any CC-licensed resource that loaded button images from Creative Commons! And it could have been worse. If I had used the HTTP version of the images, anyone in the network between me and Creative Commons would be able to track what I was reading!

Now, to be clear, Creative Commons is NOT tracking anyone. The reason my browser is sending id cookies along with button image requests is that the Creative Commons website uses Google Analytics, and Google Analytics sets a domain-wide id cookie. Google Analytics doesn't see any of this traffic- it doesn't have access to server logs. But without anyone intending it, the combination of Creative Commons, Google Analytics, and websites like mine that want to promote use of Creative Commons have conspired to build a network of web surveillance bugs BY MISTAKE.

When I inquired about this to Creative Commons, I found out they were way ahead of the issue. They've put in redirects to HTTPS version of their button images. This doesn't plug any privacy leakage, but it discourages people from using the privacy spewing HTTP versions. In addition, they'd already started to process of moving static assets like button images to a special-purpose domain. The use of this domain,, will ensure that id cookies aren't sent and nobody could use them for surveillance.

If you care about user privacy and you have a website, here's what you should do:
  1. Avoid loading images and other assets from 3rd party sites. consider self-hosting these.
  2. When you use 3rd party hosted assets, use HTTPS references only!
  3. Avoid loading static assets from domains that use Google Analytics and set id domain cookies.
For Creative Common license buttons, use the buttons from If you use the Creative Commons license chooser, replace "" in the code it makes for you with "". This will help the web respect user privacy. The buttons will also load faster, because the "" requests will get redirected there anyway.