The relative validity of the concepts of constant coefficient of friction (f) and constant interface friction shear factor (m) as quantitative indices for defining friction stresses existing in metal deformation operations has been investigated by means of the ring compression test technique. Material flow stress values have been determined by ring compression tests using mathematical formulae based upon both concepts in question. These have been compared with flow stress values obtained on the same material using the accurate Polakowski technique which is not based upon any mathematical concept, but which can only be carried out at ambient temperature and low strain rates. Flow stress values obtained by analyzing ring compression test results, using the concept of constant interface friction factor, were found to be consistently lower than the values obtained using the constant coefficient of friction concept, and in good agreement with results obtained using the Polakowski technique. For this reason, it has been concluded that the concept of constant interface shear factor (m) is the more realistic as a quantitative index for friction stresses existing in metalworking processes.