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Sergio Aguero missed the Manchester derby , and was banned for two more games, because his elbow on West Ham player Winston Reid was deemed “brutal” .

In that same derby, Zlatan Ibrahimovic charged, elbow-first, into Nicolas Otamendi, catching him on the head, causing a cut which required bandaging.

The same FA process of reviewing that incident found nothing untoward had occurred.

Looking at video of the two incidents, it is difficult to see a difference, but three former referees who make up the FA's review panel certainly thought there was.

So we asked the FA how they decide which incidents get reviewed, and how they then come to a decision as to whether the incident warranted retrospective action.

Aguero was, quite correctly, subject to a video review under FA rules introduced three years ago to try to stop serious offences going unpunished.

Those rules have changed down the years, gradually handing more power over to retrospective reviewers.

From a time when a referee's word was law, we moved to a position where a player could be punished if video showed him committing an offence which the referee had not seen. That is, an off the ball incident.

Three years ago, the rule changed again, after a horrific challenge by Wigan's Callum McManaman on Newcastle's Massimo Haidara.

The argument was that the referee must have been looking at the incident but had not seen it well enough to realise the full significance.

So the rule changed accordingly, and that meant ref Andre Marriner, even though he was looking directly at Aguero and Reid, said he had not noticed the elbow.

The FA can then act if an incident is “brought to our attention”. That phrase is widely seen as a euphemism for “shown repeatedly on Sky Sports News” - as Aguero's misdemeanour was – or “highlighted on Match of the Day”.

The FA insist this is not the case, and that an incident can be brought to their attention from a wide range of sources – including the match referee's report, the FA's own regulation department, host broadcasters who discussed it in-game or post-match, supporters, or members of the press.

They indicated that Ibrahimovic's challenge on Otamendi WAS reviewed by match referee Mark Clattenburg, and the verdict was that it warranted no further action.

They say that that panel was given several different angles of the incident, rather than the one which was circulated on social media.

Having seen it from various angles, they decided that the incident warranted no further action.

The FA stressed that their policy on 'not-seen' incidents is agreed by the whole game, including the leagues, clubs, players' union PFA, League Managers Association and the refs' organisation PGMOL.

So the conclusion is that, as with any decision, it boils down to human perception – although City fans will still wonder how Zlatan's challenge was deemed fair.