Sadly, there is no science of coaching. Professional coaching at its best is largely based on rules of thumb, heuristics, trial and error, a limited amount of psychology, a bit of wisdom and a dash of hope.
Coaching is the proper domain of empirical psychology. The field must move towards relying on empirically validated methods for helping people identify goals, motivating them to achieve those goals, and supplying the training and support they need to get there.
When universities fully invest in the study of coaching as an applied, interdisciplinary profession, things will dramatically change. You will see a shakeout in the coaching world, and clients will be able to more reliably evaluate and choose good coaches.
Most coaching training currently takes place in private institutes, using methods inherited from self-help movements, new age philosophy, and psychological techniques that were popular in the 1960’s and 1970’s. The only qualification for enrolling in coaching training is a credit card. I expect that most of these schools, and the methods they use, will vanish when coaching is established on empirical foundations. I can’t wait.