This is an interesting question since psychotherapy is incredibly personal and can be hard to quantify. That is why it is important for goals and targets to be set in therapy, so that there is then a benchmark to measure outcomes by. Setting goals is something to consider with your therapist in the initial stages.
Questionnaires taken at the beginning, end and review points of therapy can be helpful too, such as the Patient Health Questionnaire (PHQ-9), Generalised Anxiety Disorder Scale (GAD-7) and Strengths and Difficulties Questionnaire (SDQ), scores can be totalled for comparisons during different phases of therapy.
Larger scale research into psychotherapy and counselling is also undertaken, sometimes taking years to compile and assess. Often this type of research uses meta-analysis, a statistical technique for combining data from multiple studies on a particular topic to produce outcomes, one example is “The Evidence for What Makes Psychotherapy Work” (Wampold and Imel, 2015).
There might be more quantitive data available if therapists had the time to commit to research, however we often have to train for years and due to our commitments to practicing therapy, there may not be the time to prioritise research. We want to do the important work we trained to do (although research is clearly important too).
Key to therapy outcomes is the therapist-client relationship, research suggest that this is vital for the success of therapy (Barkham et al, 2018, Orlinsky, Rønnestad, & Willutzki, 2004) and I would agree with this.
As the UKCP suggests; “Safe and not judged. Understood. Heard. The right therapist will help you to feel like this so that you can find better ways to cope with your problems and improve your emotional wellbeing”.