While it will make my argument less strong in general, individual errors, like aerial duals or 50/50's won are entirely subjective. Ask 50 people to watch 10 games and give you the number of 50/50's a player won, and you'd get 50 different answers, same goes for individual errors. So often a player will be "guilty" of an individual error because someone else made a mistake, put them in a vunerable position and they then make an error that only happened because of the other persons mistake.
But in general, people are talking about an increased quality defence for Arsenal, and its bogus, we conceded less because some seasons you concede less, some more, thats luck, thats down to the striker on the opposition having bad games, or not getting penalties they deserved, or pure luck. If a GK has 8 great games and 30 crap games, one season the 8 will make all the difference in the world, another they'll be in games where you concede 5 instead of 8 goals and it makes no difference.
Arsenal have been worse at the back and Kos isn't at all bad, Mert is a bit of a joke, Sagna has gotten worse(but not much), Gibbs is rubbish defensively and cost us goal after goal what, around/before xmas and his injury. Obviously to a degree that individual error as I said is subjective and I don't trust any of the guys who do stats to get half of them right, hence it weakens my argument, but in general Arsenal have had a higher count of individual errors, and for quite some time.
Verm often looked terrible, because he was rushing back to make up for Mert rushing out and absolutely fluffing a tackle, Verm made MANY excellent last ditch tackles in those situations, and many mistakes, that happens when you're covering others mistakes.
Kos, I think he gets done for less individual errors because he has a habit of rushing out like Mert, but unlike Mert, he usually demolishes a player to kill the attack
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In fact, he does that an awful lot, better than coming out tamely like Mert does, who runs out of position, fluffs his tackle, and leaves us incredibly vunerable.
Its been this way for 5-6 years now, at least, and it hasn't improved this year under Bould
If we need a new defensive coach, new players, or just for Wenger to let go, who knows. Many rumours from ex players/general people at the club who say Bould isn't being allowed to coach how he wants, and its Wenger who tells him what to do, which would explain why absolutely no change in the defensive style despite different coaches, in which case Bould without Wengers tight leash around his neck might work, maybe Bould just isn't good enough.