title: Equations for converting scores between depression scales (MÅDRS, SRS, PHQ-9 and BDI-II): good statistical, but weak idiographic, validity.
abstract: BACKGROUND
  A variety of depression rating scales are currently used in clinical and research practice. As these scales are generally thought to correlate well, there may be some benefit in deriving equations to translate the score on one scale to that on another.
  
  
  METHODS
  Using pairwise ratings, we compared the Montgomery-Åsberg Depression Rating Scale (MÅDRS), Beck Depression Inventory II (BDI-II), Patient Health Questionnaire (PHQ-9) and Zung Self Rated Scale (SRS). The primary comparisons of interest were as follows: PHQ-9 and MÅDRS, PHQ-9 and BDI-II, SRS and MÅDRS, and SRS and PHQ-9. One hundred and fifty pairs of ratings were collected for each of these four comparisons, and these were used in a regression analysis to generate a predictive equation between scale pairs. The predictive equations were then tested for accuracy by using novel data.
  
  
  RESULTS
  There was a reasonably strong correlation between scales. Simple regression equations describe the relationships between certain permutations of the BDI-II, PHQ-9, SRS and MÅDRS well enough to allow the score on one scale to be estimated from the score on another. On average, the equations work well. However, when used to predict individual cases, they often fail.
  
  
  CONCLUSIONS
  It does appear that conversion equations for depression scores are precise enough to be of use when applied to averages; these might aid comparison of data across different studies, meta-analysis or instrument selection for clinical trials. But regarding scale translation at the idiographic level, the variability we have observed is so great that severe doubt is cast on the validity of such an approach.
authors: Hertfordshire Partnership University Foundation NHS Trust, QEII Hospital, Mental Health Unit, Welwyn Garden City, Hertfordshire.
authors: Chris J
authors: CJ
authors: Hawley
authors: Tim M
authors: TM
authors: Gale
authors: Paul St John
authors: PS
authors: Smith
authors: Sanjay
authors: S
authors: Jain
authors: Ako
authors: A
authors: Farag
authors: Raj
authors: R
authors: Kondan
authors: Cerian
authors: C
authors: Avent
authors: Juliet
authors: J
authors: Graham
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D003863:Depression / Q000175:diagnosis*; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D011569:Psychiatric Status Rating Scales; D012044:Regression Analysis; D011795:Surveys and Questionnaires; D055815:Young Adult
keywords: BDI; MÅDRS; PHQ; SRS; correlation; depression; equation; rating scale