If you were working with a lawyer, chances are he or she could tell you the odds you'll get a positive result with your case. If you were considering going through a medical operation, the surgeon would likely give you an estimate of the chance of success that your physical issue will improve after a given procedure or surgery. However, if you were to ask your average therapist how effective they are, they'll likely just give you their best guess or say something to the effect of "My clients are often satisfied with their care." Even if they tell you their past clients are overwhelmingly satisfied, that's actually not comforting, as research shows that client satisfaction is a poor predictor of actual changes in a client's well-being (Blais et al., 2002). Satisfaction with your therapy doesn't necessarily mean the therapy is helping you change. Not only that, without the aid of standardized instruments, both expert and recently trained therapists consistently make incorrect judgments as to whether their clients are improving or not (Lambert, 2015).
It's unfortunate that therapists frequently do not keep up with their outcomes systematically. That's not right, and it's unfair to you because your well-being is at stake. It's a shame that it's difficult to be completely sure about who you are getting involved with especially in regards to something as important as therapy. You deserve to be an informed consumer who gets the most for your time and money.
Well, here's my effectiveness statistics, but before you can make sense of them, let me give you a brief explanation to help you interpret things.
Therapists measure their success rate with what's called an effect size. There are many different units of measurement that can be used to meaningfully quantify an effect size, but the most common one for therapy is called Cohen's d. Decades of research has confirmed that on average, therapy has .80 units in Cohen's d effectiveness (Wampold, 2001). This means that about 78.8% of the people that go through therapy with the average therapist are better off than the person that remains untreated. (For an interactive way to understand Cohen's d treatment effects, check out this site and adjust the “slide me" bar yourself.)
So, where do I stand? I have been tracking my therapy outcomes with almost every client I've seen since 2011. My lifetime therapy effectiveness, in terms of Cohen's d, is 1.28. This means that about 90% of the people that have ever gone through therapy with me have greater levels of well-being than those that don't go to therapy and rely on their own devices.