A model for setting analytical quality specifications and design of control for measurements on the ordinal scale

A model for characterization of measurements on the ordinal scale is presen ted. It is based on transformation of the calculated fractions (fractiles) of positives from measurements on samples with known concentrations to a pr obit-natural log (probit-ln) scale. Such measurements could be made by othe r methods on ratio or difference scales but, for convenience (for exam pie for speed or low cost), are measured on the ordinal scale by "simple" metho ds. The model is examined, and verified, using three examples from publishe d data (haemoglobin, glucose, and leukocytes) and an external quality asses sment survey on measurements of streptococcus. We show that it is possible to obtain reliable analytical quality specifications and to establish desig n of control systems for measurements on the ordinal scale. It is concluded that the presented probit-ln model for the ordinal scale is a tool which c an improve and facilitate (i) characterizing methods with measurements on t he ordinal scale, (ii) defining analytical quality specifications, (iii) de signing external assessment as well as internal control schemes, (iv) valid ation of methods with measurements on the ordinal scale according to the an alytical quality specifications, and further, (v) reduction of the number o f samples required for method validation and the number of replicate measur ements needed.