IGN cites data from Valve's Steam service that reveals how many players complete the game in question on Steam.
“Statistically, most players don’t finish games. We’ve all seen numbers that say something like a third [do finish games], on average," Microsoft Game Studios’ Design Lead Richard Rouse III said later in the presentation.
The Walking Dead: Season 1, Episode 1 - 66%
Mass Effect 2 - 56%
BioShock Infinite - 53%
Batman: Arkham City - 47%
Portal - 47%
Mass Effect 3 - 42%
The Walking Dead: Season 1, Episode 5 - 39%
The Elder Scrolls V: Skyrim - 32%
Borderlands 2 - 30%
In slightly related news, Eurogamer YouTube Editor Ian Higton shows how he speed-run finished Metal Gear Solid 5: Ground Zeroes in a mere 10 minutes.