Observer variation, dysplasia grading, and HPV typing: a review

Squamous dysplasia of the cervix is a morphologic continuum that is divided into a number of categories. When the severity of a morphologic abnormality is assessed, whether in a biopsy sample or in an exfoliative smear, there can be significant observer variation. The statistical quantification of this variation, with particular reference to the kappa statistic and the influence of the number of categories utilized on the kappa statistic, is discussed. The contribution of information theory to the understanding of the reasons for observer variation is explored. The use of a binary (Bethesda) system of classification of squamous dysplasia and its validation by comparing consensus diagnoses with human papilloma virus type both in biopsy samples and in smears is reviewed.