Klaske van Sluis

30 2.3. Results Figure 2.3: Left violin plot displaying the distribution of the mean HNR outcomes in eight studies [27, 32, 33, 35, 38, 41, 44, 52]. ES n =86, TES n =82, H n =90. Right violin plot displaying the distribution of the mean MPT outcomes in eight studies [35, 37, 38, 38, 40, 41, 45, 49, 52]. ES n =84, TES n =123, H n =55. Abbreviations: ES = esophageal speakers; TES = tracheoesophageal speakers; H=Healty speaker; ELS = electrolarynx speakers; HNR = harmonics to noise ratio; MPT = maximum phonation time. Jitter Comparison of jitter values between groups were made in eight studies [27, 32, 35, 38, 42, 44, 49, 52] ( n =294). One study was categorized as level A [32] and seven studies as level B [27, 35, 38, 42, 44, 49, 52]. Low jitter value is related to better voice quality [9, 20]. Lowest values for jitter are found in the group of healthy speakers ( n =6 [27, 32, 38, 42, 44, 49], Figure 4). A significant difference between healthy speakers and the groups of substitute voice speakers is found in four studies, one of which is listed a level A study [32], and three as level B [27, 38, 44] (Table 2.3). For the groups of substitute voice speakers lower jitter values were found for TES compared to ES, but these outcomes were not significant ( n =5 [27, 35, 42, 49, 52], Table 2.3). Shimmer Six studies [27, 32, 38, 44, 49, 52] ( n =245) reported on shimmer values, of which one study was categorized as level A [32], five as level B [27, 38, 44, 49, 52]. Outcomes for shimmer in percentage were compared. For two studies [49, 52], shimmer outcomes were presented in shimmer decibel. For this systematic review, these data were recalculated to percentages [49, 52]. A low shimmer value is related to a better voice quality [9, 20]. In the level A study of Shim et al. [32] significantly better shimmer outcomes are reported for healthy speakers compared to TES (Table 2.3). Shimmer values for all groups ranged from 0.4% - 18.4%. The value of 53.6% shimmer for ES, is considered to be an outlier [44]. It seems that based on shimmer, healthy speech can be rated as superior compared to ES and TES ( n =5 [27, 32, 38, 44, 49], Table 2.3, Figure 4). No