1. Validity

This chapter covers all elements of the validity of diagnostic tests relevant for physiotherapists whenever one reads a research paper, or searches the literature for clinical relevant diagnostic tools necessary to set a diagnosis or develop a treatment plan. In this chapter I explain the different forms of validity, such as sensitivity, specificity and predictive value. Furthermore, I explain concepts such as a likelihood ratio, diagnostic odds ratios and ROC-curves (receiver operator curves), although these are more relevant for research than clinical practice.