What is the independent variable in the following hypothesis?

The incidence of pain will be the independent variable in a study where a given intervention seeks to establish whether or not that intervention has the effect of lessening pain. Pain is a subjective experience and therefore the only way of measuring this might be to use the patients' own report of the degree of pain being felt.

This kind of research is hampered by the inability to measure pain objectively. However, it could be argued that if the patient himself or herself believes they are suffering less with Music or whatever the intervention might be, then the intervention is worthwhile.