You also have to account for non-smoking children who would've switched to cigarettes but switched to e-cigs instead. An interesting question is if e-cigs caus

You also have to account for non-smoking children who would've switched to cigarettes but switched to e-cigs instead. An interesting question is if e-cigs cause N children to use them who wouldn't have otherwise, but also means that M children who would used cigarettes instead use e-cigs, what is the function to determine if it is considered a net positive?