64 All swallowing parameters (number of swallows, duration, swallowing volume, swallowing capacity, and swallowing speed) showed good to excellent test-retest correlations for patients with HNC (ICC>0.75), and moderate to excellent correlations for healthy subjects (ICC>0.70) (Table 3). The SEM values indicated that there is an expected random variation in the different parameters of 5.9-19.1% for patients with HNC, and of 4.0-13.8% for healthy subjects. The SDC values indicated that the difference between two tests needs to be higher than this SDC value to be considered a true change in swallowing, which is not caused by a measurement uncertainty. Therefore, the difference for the different parameters needs to be higher than 16.5-52.8% for patients with HNC, and 11.1-38.2% for healthy subjects. The Bland-Altman plots (Figure 1 and 2) showed that 95% of the data lie between the Limits of Agreement, indicating no systematic variation in performance between two measurements. The Wilcoxon Signed Ranks test showed no significant differences between test and retest for all swallowing parameters except swallowing volume in patients with HNC. The KruskalWallis test showed a significant effect for age for all parameters (number of swallows, p<0.001, duration, p<0.001, swallowing volume, p=0.001, swallowing capacity, p<0.001, swallowing speed, p=0.005). Number of swallows and duration increase with age, and swallowing volume, swallowing capacity, and swallowing speed decrease with increasing age. In addition, a significant effect was found for sex for number of swallows (p=0.033) and swallowing volume (p=0.044). Women need a higher number of swallows and have a lower swallowing volume in comparison to men. Duration (p=0.257), swallowing capacity (p=0.257) and swallowing speed (p=0.373) did not show an affect for sex.
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