I think Y va is talking specifically about a signing pushing a team to the title. Many of the examples given are of players pushing their teams out of relegation, into the top 4 etc. Whilst the improvement and impact is discernible in all these cases, those who push a team to the title may not be so clear. Reason being that most of these teams who go on to win the title are usually good enough to win the league with or without a signing in January.
On the flip side, if we expand this debate to impact of January signings pushing their team to whatever goals they want to achieve, there are a number of examples already given. Although there isn't much evidence of a January signing pushing his team to the title (for probably the same reason that they were already good enough to start with), whilst those who never really had a chance would need some super star signings that aren't available in January to compete in the first place.
However it is a bit strange to ignore all the players who have made a difference in the examples given to the teams that bought them in January and come to the conclusion that teams that are competing and may be good enough either way could not use the benefit from the marginal improvements January signings can make to get them over the line.
Not saying that a player will win them 10 points on his own but he could win them a game here and there on his own or stabilise the defence etc. Equally he could be the difference between points kept when the first team are fit and lost during periods of injuries, suspensions or fatigue is he is a squad player.
So yes, there may be no correlation between January signing and winning the title for teams that were never really in it in the first place, especially due to the lack of the level of players needed for that kind of a job. But there are plenty of examples of players adding that bit of extra to help push their team over the line and on to the objective they were competing for. Objectives they genuinely had a chance of achieving, which may well have happened in any case through internal improvements, but were made a bit easier with that January signing.
