K6 AND K10 COMPARISON
The K6 is a truncated version of the K10 in which four questions are not used: the ‘tired out for no good reason’ question, and the three ‘if not none of the time’ questions (see table B, below). Given its shorter nature, the K6 cannot entirely rely on the validation of the instrument on which it was based. Omitting or modifying questions from an accepted psychometric instrument may produce a reduction in efficacy.
According to Furukawa et al (2003), the K10 performed marginally better than the K6 in screening for Composite International Diagnostics Interview (CIDI) and Diagnostic and Statistical Manual for Mental Disorders, Fourth Edition (DSM-IV) mood and anxiety disorders. However, the K6 is preferred in screening for DSM-IV mood or anxiety disorders because of its brevity and consistency across subsamples.
A small validation study carried out in Boston found evidence that the six-question scale is at least as sensitive as the ten-question scale for the purpose of discriminating between cases and non-cases of serious mental illness (Harvard School of Medicine, 2005). K6 validation studies were also carried out in a number of countries throughout the world, which uniformly found that the K6 had very good concordance with independent clinical ratings of serious mental illness (Kessler et al, 2010). The K6 is now included in the core of the US National Health Interview Survey (NHIS) and the National Household Survey on Drug Abuse (Kessler et al, 2002).
Table B: K6
Q | K6 – 2001 version | 2001 K10 Q |
 | During the past 30 days: | |
1 | about how often did you feel nervous? | 2 |
2 | about how often did you feel hopeless? | 4 |
3 | about how often did you feel restless or fidgety? | 5 |
4 | about how often did you feel so depressed that nothing could cheer you up? | 8 |
5 | about how often did you feel that everything was an effort? | 9 |
6 | about how often did you feel worthless? | 10 |
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