When Chelsea this week issued their manager with a vote of confidence for the first time in the Roman Abramovich era, they probably did not realise the implications of what they were doing. On the face of it their actions were a show of support for Jose Mourinho: it was unusual for them to publicly back their increasingly beleaguered manager rather than simply discard him like they had done with so many before.The closest the very private Abramovich had previously come to a vote of confidence was in November 2011, when he told Andre Villas-Boas he had his support, only to dismiss him 100 days later.That is generally what can be expected of managers who are given the dreaded "vote of confidence" and in public, too.Very rarely indeed do managers who are backed in public ever survive for a significant amount of time. Often they are removed within a matter of weeks.A sample of 16 recent votes of confidence reveals that the average amount of time that ultimately sacked managers last at that club is only 48.4 days. Were Mourinho to survive that long, it would take him through to November 22, a day after facing Norwich at home, where a poor result may force Abramovich's hand.Brendan Rodgers lasted only 57 days after he was given his board's backing on the eve of this season, while earlier this year Ian Holloway was afforded only 11 more days at Millwall...