Essentials of Biostatistics. 6. Reference values in medicine and validity of diagnostic tests

Abstract

We earlier defined biostatistics as the science of management of uncertainties in health and disease(1). In the previous articles of this series(2,3), we also included the numerical and graphical methods to summarize data. The sheet anchor of all this is the measure-ment because it is through measurements that quantities are obtained, and variations and uncertainties aptly studied. Thus, biostatistics can also be perceived as the quantitative aspects of health and disease. The qualitative aspects such as symptoms and relief are more important in evaluating an individual's health but that is the domain of a clinician. Some quantitative measures commonly used for assessing the health of a child are Apgar score, respiration rate, body temperature, and weight for age. Measurements are used not only in assessing the levels of health and its various components but also in assessing the disease severity. One example is Yale's observation scale (4) which is used to identify serious illness in febrile children. Measurements are also used in establishing and interpreting the reference values of various medical parameters, in evaluating probailities in diagnosis and patient management, in assessing the validity of medical tools, etc. The usual practice in medicine is to evaluate various parameters of a subject against a single or a range of reference values. The methodology generally used to delineate such reference values as well as their implications are discussed in Section 6.1. Because of variations and un-certainties, this assessment is done in terms of probabilities. These are discussed in Section 6.2. Section 6.3 is on assessment of validity of diagnostic tests.