My favorite explanation of the perils of the transfer market comes from one of the guys who best knows how to beat it. At last year's StatsBomb Conference, Liverpool's director of research, Ian Graham, put forth his theory of player acquisition. Roughly, Graham said, there are six reasons why a transfer might fail:
The player isn't as good as you thought
The player doesn't fit your style
The player is played out of position
The manager doesn't like the player
The player has injury and/or personal issues
There's a better player already on the roster
And even if you're 90% sure that each of those six individual factors won't be an issue, those minor degrees of uncertainty still add up to only a 53% chance (0.9 x 0.9 x 0.9 x 0.9 x 0.9) of the transfer working out. When you realize that most transfers don't carry that 90% certainty factor across all six facets, it's pretty easy to understand why playing the transfer market is so often a loser's game.
Last month, my colleague Gabriele Marcotti looked at the 10 most expensive transfers by position and concluded that "19 of the 41 deals ... are ones clubs would not want to repeat with the benefit of hindsight (and if you take away goalkeepers, it's 17 of 31, more than half)." I want to build on that idea, using Graham's framework, to see if we can understand why so many big-money transfers -- ones backed by an amount of cash that suggests a much higher degree of certainty than your average deal -- have seemed to fail.