Some lol player on reddit did a statistical analysis of over 250k games from April-June 2011 and posted the findings:
http://www.reddit.com/r/leagueoflege...torical_games/
Trolls and Win Rates:
Criteria Win Rate
Having one feeder (KDA score < -7) on the team 7.8%
Having two or more feeders on the team 2.5%
Having an inactive player (0 kills, 0 assists, < 10 cs) on the team 16.7%
Frequency of Trolls:
Troll Type % of Population
Completely AFK 0.15%
Nonfeeding inactive 0.32%
Feeder 1.95%
Any of the above 2.26%
Comments: A feeding troll is even worse than an inactive troll. If 2.26% of population are trolls, there's a 8.74% chance of having a troll on your team (assuming you’re not a troll), and 10.8% chance of a troll on the other team. =( [Thanks thelordpsy for the correction]