14 proven to be highly reliable in patients with HNC.47 Objective swallowing performance can be measured with, for example, Fibreoptic Endoscopic Evaluation of Swallowing (FEES), or in a non-invasive and fast manner with minimal equipment using a 100 mL Water Swallow Test (WST).32,48,49 Measures of objective salivary flow rate from parotid and submandibular glands have been used for years to determine the dose response relationship between RT dose and degree of hyposalivation or sticky saliva.38,39,43,50,51 To obtain high quality measures, it is important to assess measurement properties such as reliability.52 The reliability of a test measures the degree to which a certain test produces stable and consistent results. This can be achieved by, e.g., a test-retest with corresponding intraclass correlation coefficient (ICC) (for continuous data). In addition, inter- or intra- rater differences can be assessed. During each test, a measurement error is thought to occur, either by the measurement instrument itself, the measurement situation, the person taking the test, or the person being tested. The general idea is that, the lower the measurement error, the higher the reliability and thus the quality of the measurement.52 Subjective oral functioning can be measured with several validated questionnaires.53 The European Organization for Research and Treatment of Cancer Quality of Life core Questionnaire, Head and Neck module (EORTC QLQ-H&N35) was especially developed to measure HNC specific problems and addresses different items of oral functioning.54-56 The Dutch version of the Swallow Quality of Life questionnaire (SWAL-QOL) was developed to address swallowing and food processing related problems.57,58 The Groningen RadiationInduced Xerostomia (GRIX) was developed to observe xerostomia and sticky saliva during day and night.59