That's not how the legions worked. The men assembled three ranks deep. The first rank was comprised of those with middling experience, who had seen combat before. The rear rank was the senior veterans. The cherries with no experience went in the middle, both to be reassured by the E4 mafia laying skunion in the front rank, and to be prodded on by the very experienced guys behind them.
In that kind of setting I imagine the low performers would sometimes be likeliest to survive, if they broke rank to run away. I imagine the lots would come out if everyone gave an honest effort.