People buy stuff when it goes on sale. This simple and intuitive business rule has proven itself time and again, and especially for HD DVD. A couple of weeks ago, the now famous $99 deal for an HD DVD player appeared at Wal-mart and was soon matched in whole or in spirit by other retailers. The result was the addition of several tens of thousands of new HD DVD owners.

In the last couple of weeks, those new HD DVD owners started looking for movies to buy, and retailers have slowly responded with various sales on time for Black Friday. Heading into the heart of the Christmas shopping season, people will be looking for HD DVD titles for themselves as well as for gifts, and it’s likely that we’ll see even more deals on movies. As mentioned earlier, when desirable stuff goes on sale, we can predict more business for the merchants who put on those promotions.

While not a perfect indication of the overall market, Amazon.com’s sales data provides some insight into trends that are likely to be happening at other retailers. It’s not a perfect predictor or estimator because Amazon often has a sale that’s not directly comparable to what’s offered at a competitor. For example, the “$50 Bourne Trilogy” and “47% off Universal movies” deals are their own creation. There are also cases where Amazon is slow to offer a promotion that’s already been available elsewhere, such as when Best Buy and Circuit City ran their own “$199 HD-A3 and 10 movies” as well as “buy 1 get 1″ or “buy 2 get 1″ offers days or weeks before Amazon took notice.

Amazon is not the perfect indicator, but they’re not a bad reference point either.

The guys at HdGameDb.com plugged into Amazon’s sales data for the purpose of comparing stats about video game sales, and the folks at High-Def Digest are currently using this technology to track sales of high-definition discs — you can even make a chart of this data. If you’ve read this far along, then you can guess what I’m about to report, so perhaps I’ll just skip to a picture instead of a thousand words:

This chart is adapted from the real thing at HighDefDigest.com, and shows the HD DVD sales rankings for the last two weeks ending this Thanksgiving weekend. The Blu-ray data is removed because I didn’t want to turn this into an argument with supporters of that format. Clicking on the graphic will take you to the most recent chart, which may look different than the snapshot presented here.

The point I’d like to make is that HD DVD owners have responded favorably, and predictably, each time Amazon has come out with an interesting deal. The first bit of notable activity happened around the time that a bunch of Universal titles were offered at nearly half off. This activity level was sustained as the Bourne Trilogy and HD-A3 deals came online a couple of days later, and combining it with the “Buy 2 Get a Third Free” deal helped HD DVD sales break the top 1,000 ranking, getting into the top 600, for the first time since this charting has been available.

Blu-ray sales have been just as predictable. In fact, Amazon has had better and more Blu-ray disc promotions than for HD DVD, and Blu-ray discs have done especially well during those times.

As good as it is to break these sales barriers, what’s more significant is that there’s a general upward trend compared to much earlier in the year. Combined with the thousands and thousands of new HD DVD users and their predictable quest for movies, software sales for HD DVD should be extremely promising in the months ahead — provided the software pricing is favorable.

I’m not sure how much of it depends on the studios and how much is on the retailers, but the way to keep this trend going is for both of them to keep the software deals coming.

It’s fair to say that all retailers who put out reasonably good HD DVD promotions can expect to report the same kinds of healthy results for the format as Amazon has. This means that the format is indeed very popular and viable, and there’s a lot less reason to worry about getting into HD DVD.

Here are links to two of the Amazon promotions responsible for most of that chart (the $199 HD-A3 and 10 movies deal, and “Buy 2 Get 1 Free” are over):

• 47% off on Universal movies
• Bourne Trilogy for $50

UPDATE: Amazon has the HD-A3 for $179 again.