Pell 1973.

Methods	CBA study Prospective
Participants	Various workers n = 1572 n = 628 < 20 dB hearing loss at entrance n = 559 15‐35 dB hearing loss at entrance n = 385 > 40 dB hearing loss at entrance USA 1 company
Interventions	Intervention: HLPP mainly hearing protection; n = 399 Control: non‐exposed colleagues; n = 1173
Outcomes	Average change in hearing thresholds last‐entrance measurement at 0.5, 1, 2, 3, 4, 6 kHz 5‐year follow‐up
Notes	Long‐term Comparability ‐ intervention/control: average age: intervention ‐ 34 years; control ‐ 43 years hearing: stratified according to HL at start
Risk of bias
Bias	Authors' judgement	Support for judgement
14. Blinding (subjects)	High risk	not blinded
15. Blinding (outcome assessors)	High risk	not blinded
16. Retrospective unplanned subgroup analyses	Low risk	no unplanned analyses
17. Follow‐up	Low risk	5‐year follow‐up
18. Statistical tests	High risk	ANOVA
19. Compliance	Unclear risk	not reported
20. Outcome measures	Low risk	audiometry quality reported
21. Selection bias (population)	Low risk	same factory
22. Selection bias (time)	Low risk	same time
23. Randomization	High risk	not randomised
24. Allocation concealment	Unclear risk	not randomised, not applicable
25. Adjustment for confounding	High risk	difference in mean age was 10 years
26. Incomplete outcome data	Unclear risk	not reported