Information and observer disagreement in histopathology

Concepts from information theory are used to assess the process of ordinal classification of histopathological continua such as degree of dysplasia or degree of differentiation. A working model is used to show that reducing the number of categories leads to an increase in inter-observer agreement, as measured by kappa statistics, but a decrease in information transmitted. It is argued that the prime role of histopathologists is to pass information, obtained from visual patterns, to the clinician and therefore more categories, rather than fewer, should be used. It is suggested that judgements of continua should be made on 100 point scales and quoted together with 95% confidence limits; and that the judgements of different observers should be averaged to form a reference range.