Whilst considerable attention has focused on improving the detection of depression, assessment of its severity is also important in guiding treatment decisions. The PHQ-9 screens specifically for signs or symptoms of depression. Nine specific questions are built on the criteria for depression that were identified in the Diagnostic and Statistical Manual of Mental Disorders, 4th edition (DSM-IV).
Research studies have examined the consistency and validity of the PHQ-9 and have repeatedly demonstrated that the PHQ-9 is an effective way to measure depression. Studies have also verified that the PHQ-9 accurately identifies the presence of depression, as well as measures the severity of depression. The attached article examines the validity of the measure:
Link to article: ThePHQ9
Alternative research: UtilityofPHQ9