"Our industry is hit driven," Greg Short told an audience of game marketers. "But unfortunately, despite all the work we put into them, they don't always bring home the bacon," said the chairman of Electronic Entertainment Design and Research. He offered statistics, saying that only one in five games is a success.

Marketers are responsible for making decisions that impact how successful that game is. "But what are you decisions being based on?" asked Short. "To make the most of the knowledge and creativity that your staff possess, you need your stats and your facts earlier."

EEDAR has specialized in objective-based data that classifies games in minute detail. "We don't do any consumer polls," explains Short, because "We use that data over and over again. And you need to be able to form your own opinion, instead of using someone else's."

After a detailed discussion of the importance – and newness – of player achievements and how they could creatively improve game sales, the company's president, Geoffrey Zatkin, showed some never-publicly-presented numbers, dealing with game demos and game trailers.

"Not all advertisements increase sales," noted Zatkin, saying that while many games have downloads available to players, "different types of downloadable content have different results."

Cumulative unit sales increase with both a trailer and a demo over a game that has neither. Having a demo only causes those numbers to fall bellow the demo-trailer combination. "Having the demo is worthless than having nothing at all," said Zaktin.

But the graph soars when just a trailer is available for download. "On both consoles, having the demo, that does help the decay rate," continues Zatkin. This has led them to hypothesize that publishers should do a trailer at launch, and maybe launch the demo three months later. "A bad demo can keep people form buying your game," he added.

This reduces the impact on the development, states Zatkin. "And you're not impacting their critical timeline."

While it's true across the board, it does trend differently by genre. For example, says Zatkin, for sports games on the Xbox 360, "trailers have less of an impact."

"The latest evolution of game hardware has given all of you the chance to excel," concluded Short. "And we're in the game industry, so have fun."

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The bolded part is what I highlighted as the true problem rather then demos in general.  I can understand not having a demo and rather a video since you can trick people into thinking it is a good game by showing off of the graphics etc.  Bottomline is you have to make good demos and I say have them available with the game launch or a week before etc.  I always hated how EA released their games with no reviews for like a week so they can sell as many as they can before the crappy reviews start coming out.  Is basically the same way with demos.  Of course I may be baised as I just love demos and has actually lead me to buy a few games because of them.