46 In previous research, different tests have been developed to measure mastication, such as comminution methods, sieve and optical scanning methods, gummy jelly as test food, and mixing ability methods.13 Construct validity was positive in one method measuring mixing ability to test oral function: the Mixing Ability Test (MAT).12 The MAT was specifically developed for patients with HNC,7,12,14 consisting of a relatively soft material (wax), to make sure patients with compromised mastication would still be able to perform this test. This test has proven to be sensitive in measuring mastication in adults with dental deficits, and children with cerebral palsy.15,16 However, reliability of this test has not been evaluated in patients with HNC yet. The aim of this study was therefore to provide insight in the reliability of the MAT, by investigating test-retest reproducibility, standard error of measurement, smallest detectable change, and limits of agreement in patients with HNC. In order to make a comparison between patients and healthy subjects, reliability of the MAT was tested in healthy controls as well. Methods Patients were included when they were 18 years or older, were diagnosed with oral, oropharyngeal, hypopharyngeal, laryngeal, or unknown primary HNC and were treated with a curative intent at the University Medical Center Utrecht (UMCU), the Netherlands between September 2016 and June 2018. Patients with recurrent or residual disease, cognitive impairments, and patients having trouble understanding or reading the Dutch language were excluded. Healthy controls were recruited through a poster at the outpatient clinic, between November 2018 and February 2019. Healthy controls were included when they were 18 years or older. The study protocol for patients with HNC was approved by the Medical Ethics Committee of the Netherlands (NL45051.029.13), which is part of the NETQUBIC research.17 The study protocol for healthy controls was approved by the Medical Ethics Committee of the UMCU (18/701). Patient data about age, sex, tumor stage,18 tumor location, treatment, number of teeth (maximal 32 teeth) and number of occlusal units (maximal 16 units)19 were collected. In addition, data about age, sex, number of teeth and number of occlusal units were collected for healthy controls. All participants signed informed consent before participation. Mixing Ability Test The Mixing Ability Test (MAT) consists of two layers of wax, with the colors red and blue (Plasticine modelling wax, non-toxic DIN EN-71, art. nos. crimson 52801 and blue 52809, Stockmar, Kalten Kirchen, Germany).7,12,14,20 The total thickness is 3 mm, with a diameter of 30 mm. The outcome variable is called the Mixing Ability Index (MAI), and ranges between 5-30, where a lower MAI score implies a better mixed tablet and better masticatory