Barnett (1995,Barnett, P. W. and Knight, R.D. (1995). "The Common Intelligibility Scale", Proc. I.O.A. Vol 17, part 7. 1999Barnett, P. W. (1999). "Overview of speech intelligibility" Proc. I.O.A Vol 21 Part 5.) proposed to use a reference scale, the Common Intelligibility Scale (CIS), based on a mathematical relation with STI (CIS = 1 + log (STI)).
