Ian Graham is Liverpool's director of data science. He believes there are 6 main factors that could cause any new signing to fail.
Graham tells you that if your score on each of the six factors is 90%, this gives you a 53% probability of success according to the simple law of probability. Most clubs do not follow this methodology in the first place, and even if they do, they do not reach this degree of certainty before concluding a deal. Barcelona have been at the forefront of successful clubs in the transfer market.