Berg 2009.

Methods	Cluster‐randomised controlled study
Participants	Agricultural students involved in farm work n = 753 USA 34 schools
Interventions	Intervention: hearing test yearly, instruction once, 11 mailings at home, free hearing protection plus replacements, use of sound meter Control: yearly hearing tests plus questionnaires
Outcomes	STS with ≥ 10 dB loss at 2, 3, 4 kHz in either ear Median and mean thresholds at 0.5, 1, 2, 3, 4, 5, 6 kHz High‐frequency average (3, 4, 6 kHz) and low‐frequency average (0.5, 1, 2 kHz) thresholds Bulge depth 3‐year and 16‐year follow‐up
Notes	Long‐term Comparability ‐ intervention/control: mean age: intervention 14.5 years, control 14.6 years hearing: max. threshold (R or L) at 0.5 kHz intervention md 10 dB/control md 5 dB, at 1, 2, 3, 4 kHz intervention/control md 5 dB, at 6 kHz intervention md 15 dB/control md 10 dB
Risk of bias
Bias	Authors' judgement	Support for judgement
14. Blinding (subjects)	High risk	no blinding
15. Blinding (outcome assessors)	Unclear risk	not reported
16. Retrospective unplanned subgroup analyses	Low risk	no subgroup analysis
17. Follow‐up	Low risk	RCT
18. Statistical tests	Low risk	multilevel analysis
19. Compliance	Low risk	increase in hearing protector use
20. Outcome measures	Low risk	hearing thresholds, STS
21. Selection bias (population)	Low risk	RCT
22. Selection bias (time)	Low risk	RCT
23. Randomization	Unclear risk	randomisation stated but no method reported
24. Allocation concealment	Unclear risk	no information provided
25. Adjustment for confounding	Low risk	adjustment for baseline differences
26. Incomplete outcome data	Low risk	< 20%