Table 1.

Research questions and outcomes of interest

Research questions	Which comparative acoustic, perceptual, and PROs are available for TES, ES and ELS after TL? How do outcomes of TES, ES and ELS relate to those of normal laryngeal speech? What outcomes are favorable in which rehabilitation method?
Primary outcomes
Acoustic	F 0: fundamental frequency, a result of the rate of vibration of the (neo) glottis [15] HNR: harmonics to noise ratio, ratio between the total energy of the periodic voice signal and the energy of noise components [15, 17, 20] %voiced: the percentage voicedness [9, 16, 17]
Perceptual	Voice quality: impression of the overall voice quality [8, 9, 14] Intelligibility: impression of the intelligibility [8, 18]
PROs	VHI: Voice Handicap Index [13] V-RQOL: voice-related quality of life [14]
Secondary outcomes
Acoustic	Jitter: relative variability in the period-to-period frequency [20, 21] Shimmer: relative variability in the peak-to-peak amplitude [20, 21] Intensity: Loudness dB [8, 22] Spectral tilt: a comparison between low frequency energy (between 0 and 1 kHz) and high frequency energy (between 1 and 5 kHz) [21, 23] MPT: Maximum Phonation Time [22, 24]
Perceptual	GRBAS: Grade Roughness Breathiness Asthenia Strain scale assessment [18, 19] Unintended additive noise: uncontrolled noises during speech [18] Fluency: the perceived smoothness of the sound production [18] Voicing: voicing is voiced or unvoiced where it is supposed to be voiced or unvoiced [18]
PROs	EORTC QLQ-H&N35: European Organization for Research and Treatment of Cancer, Quality of Life Questionnaire module for patients with head and neck cancer 35-item version [11] EORTC QLQ-C30: European Organization for Research and Treatment of Cancer, Quality of Life Questionnaire C30 [10]

ES esophageal speakers, TES tracheoesophageal speakers, ELS electrolarynx speakers, PROs patient-reported outcomes