Using the trophy list can also skew the results. What would count as "completed the game"? Just finishing it on easy/medium, or going all out and getting the Plat (100%) and all that that entails? Some games, like The Walking Dead, you basically just have to play through the game to get the Plat (or 100% completion). Other games like The Last of Us, Ni No Kuni, Uncharted require a bit more effort or play throughs. Think after finishing TLOU it showed 5% complete. I have started a new play through on a harder difficulty, and I'm on 10% now, but still plenty to go. Ni No Kuni you can complete in 50 odd hours, but if you play normally that ends up being about a 42% or 46% according to the list. It just sometimes isn't worth it, or even fun, having to replay some games just to get a higher percentage according to your trophy list.
As for myself, I'm concentrating on a few titles which I would like to get as close to 100% as possible. Not because I have to, but because I like the challenge. Like Wlad said, if the game doesn't grip me or I loose interest halfway through I normally don't bother coming back to it later. There's just too many other quality games available or coming to keep me occupied for quite some time.
I think in this case, if I were conducting the research myself, I would track the final achievement in the story progression, be that whatever (defeated xxx or completed chapter 9) on any difficulty, this will track most players' completion of narritive based games, they are arguing that as a 1/3rd of all gamers only complete the story mode, whats the point? and I'm just arguing the stats they based their research on is questionable, and as another guy commented, you need to look how far the player got into the narrative before just clean-cutting it, narrative is very important to me as a gamer, and sure the casual kids couldn't give a shit, but that's not what I want gaming to become, half-assed stroies just cause no-one cares