I’ve only found the abstract in public spaces. I think you have to have an account to get into ResearchGate: https://www.researchgate.net/publication/346469800_Association_between_time_spent_on_smart_devices_and_change_in_refractive_error_A_1-year_prospective_observational_study_among_Hong_Kong
- The full data does look at a lot more than just <2 and >2 hours per day.
- 50% of the kids were on the smart phone 4+ hours a day.
- The outcomes were not at all linear. All groups got slightly worse on average. A straightforward interpretation of the data would say that 4+ hours is safer than 2-3 hours. The p-value of the relationship was 0.84, which doesn’t indicate a strong relationship.
- Tablet usage had a slightly tighter relationship, but not that great.
- The study only covered tablets and smartphones, not books, so this may not reflect total close-up time for the children.
- Multivariate regressions adjusted for BMI and physical activity, but I’m not sure that helps us much, as this would cancel out rather than compare some non-smartphone near work. I’d have much rather seen a component analysis on that one.
- Key finding “Children who spent more time (≥2 h per day) on smartphones, but less time(<2 h per day) on tablets showed greater negative shift in refractive error than those who spent more time on both devices. These results suggested that prolonged smartphone usage may present a higher risk of myopia progression than tablet usage.”
- "change in SER of right and left eyes might be contributed by the position of the smart device during use, but we did not collect data on the hand they commonly used "