Signal and noise in psychotherapy. The role and control of non-specific factors.

Four major categories of non-specific or common factors are systematically analysed: relationship, expectancy, reorganising, and impact factors. The placebo control method should be discarded in psychotherapy research for two reasons: (a) sham treatment controls are unworkable because common factors in treatment are manifest only when real treatment is administered; and (b) the very events which were once thought of as 'non-specific' noise are now recognised as active therapeutic factors common to many schools.