Arsenal have a problem in defence that I have noticed in a few games this season - and it was particularly evident during Sunday's draw with Liverpool.
The back four plays too high up the pitch when there is no pressure on the opposition footballer in possession.
If an opponent has the ball in space and thus plenty of time to pick his pass then the defence needs to retreat towards their own goal.
The tighter you go towards the ball the more space you leave in behind - and against a quick player like Robbie Keane, who thrives on running off a defender's shoulder, that is very dangerous.
It was no surprise that it was from a long ball over the top of midfield that Keane scored Liverpool's spectacular equaliser.
Arsenal should have dropped off before the pass had even been played but they had not done so and left enough space in behind for Keane to exploit.
The decision to hold the current line, move forward or drop off is usually made by one of the central defenders.
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Which of the two makes the decision varies depending on the situation. It might depend on which side of the field the ball is or, if one of the two is particularly vocal, he might take charge of most situations.
The defensive line that I was a part of at Arsenal was extremely vocal and that was because the process of knowing when to drop off and when to press had been drilled into us so many times that it had almost become second nature.
Early in Sunday's match a long ball found Keane in acres of space close to the Arsenal goal. It was Gael Clichy who had played him onside but the defensive unit must take collective responsibility.
Space is another issue the defence must address as a unit. There was too much of a gap between centre-halves William Gallas and Johan Djourou when the long pass upfield came in that led to Keane's goal.
Gallas and Djourou are both good defenders and should have been able to deal with Keane - it was two against one after all - but because they had too much space between them Keane was able to exploit that.
When you are part of a back four the distance between the players is crucial. Roughly speaking the distance between all four has to be equal. Sometimes they need to be narrow - perhaps five yards between them - and at other points in a game they will need to spread across the pitch. But the key thing is that they move in unison and that comes from slogging it out on the training pitch.
In Sunday's game there was another clear example of Arsenal's failure to drop off when Leiva Lucas received the ball in plenty of space in midfield. The defence should have retreated at that point, but they failed to do so and Lucas had plenty of time to pick out a run from Steven Gerrard.
It was not the fault of the Gunners defence that Lucas was in so much space, but it was their responsibility to manage the situation.
If they hold their line then they can be beaten with one incisive pass but if they drop a bit deeper it gives them some breathing space. The defence must try to slow the play down, direct the opposition into certain areas and give their midfielders the opportunity to come back into play.
Of course, there is a balance to be struck.
Keane equalised after a long ball caught out the Arsenal defence
You cannot play too deep, especially at home. If you do then you end up winning too much ball on the edge of your own box. You have then to go through the entire opposition team to reach the other side's goal.
But you need to be able to assess the situation and make the appropriate decision - and at the moment Arsenal are caught too high up the pitch too often.
Liverpool also looked to play a diagonal ball to Dirk Kuyt on the right side of midfield. Kuyt is good in the air and the Reds obviously felt that he would have a clear advantage over Clichy.
Having been a full-back of modest height I encountered plenty of situations where a team tried to exploit a height advantage.
In one sense there is not all that much you can do about it. A manager is unlikely to change personnel and as a defender what you need to do is make sure that you put in some sort of challenge. Ex-Arsenal skipper Tony Adams used to tell me to make sure I prevented the opposition player from making a clean header and he would then deal with the second ball.
I thought that Liverpool had the better of the opening half on Sunday and caught out Arsenal several times.
But after Gunners forward Emmanuel Adebayor was dismissed early in the second-half the visiting side lost the initiative and we did not see Arsenal's defence caught too high up the field.
This was probably because Arsenal had only one forward and could not press as high up the pitch.
As a consequence both the midfield and defence dropped a little deeper.
Arsenal played with great spirit and determination and were probably the better side in the second-half.
But I'm sure that Arsene Wenger will want to improve his defence's decision making when an opposition player is in possession and with time to pick out a pass.
Lee Dixon was speaking to BBC Sport's Paul Fletcher
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