you can't judge skill from outcomes in the short run if a large component of the outcome is outside of the control of the operator

see e.g. Taleb "fooled by randomness"

there's also a good "winner's game" vs "loser's game" argument, see Ellis - The Loser's Game: http://www.cfapubs.org/doi/pdf/10.2469/faj.v51.n1.1865 (PDF)