How Can You Measure an Intangible Quality in Art Therapy

How can you measure an intangible quality in art therapy, such as achieving a sense of peace or wellbeing?

A) Compare images created by people who identify as peaceful and images created by people who identify as unhappy or not at peace.
B) Survey a large sample of people about the kinds of activities that make them feel peaceful and count the number of times art therapy is listed.
C) Compare the physiological stress responses of people before and after their participation in an art therapy session that asked people to paint a peaceful scene.
D) Create detailed observations of people creating art and decide which directives most often produce a sense of peace or wellbeing.