PICO (Population, Intervention, Comparison, and Outcome) schema
| Population | General population samples of nonsmokers, any age |
| Intervention/exposure | Self-reported current or ever use of e-cigarettes, with or without nicotine |
| Comparison group | Self-reported non-use of e-cigarettes throughout the study period |
| Outcome | Self-reported ever or current use of conventional cigarettes at follow-up |