1. Controlled (non‐randomised) trials.

Study ID	Methods	Participants	Interventions	Outcomes	Results
Chan 1994	CT	25 female kindergarten teachers	1) Direct and indirect voice training (12) 2) No intervention (13)	2) Relative average perturbation, ratio of energy below 1 kHz to energy above 1 kHz, Duty Cycle (an EGG parameter)	Significant improvement in RAP when compared to no intervention
Hackworth 2007	CT	76 general and/or vocal music teachers	1) Indirect voice training (19) 2) Indirect voice training and additional information on behavioral modification/teaching techniques (11) 3) No intervention (46)	1) Daily log for 8 weeks about vocal problems, water consumption, minutes of daily vocal warm‐up, number of vocal breaks taken (complete voice rest) and talking over noise and/or use of non‐verbal commands	No significant differences between intervention and control groups
Timmermans 2004a	CT	68 students of a school for audiovisual communication	1) Direct and indirect voice training (49) 2) No intervention (19)	1) Voice Handicap Index 2) GRBAS, videolaryngostroboscopy, MPT, jitter, I‐low, Fo‐high, Dysphonia Severity Index	No significant differences between intervention and control groups
Timmermans 2004b	CT	46 students of a school for audiovisual communication (same participants as in Timmermans 2004a)	1) Direct and indirect voice training (23) 2) No intervention (23)	1) Voice Handicap Index 2) GRBAS, videolaryngostroboscopy, MPT, jitter, I‐low, Fo‐high, Dysphonia Severity Index	No significant differences between intervention and control groups

CT = controlled trial (i.e. allocation to groups was not randomised) 
 EGG = electroglottographic 
 RAP = relative average perturbation