While reading it, I was always thinking about confounding factors. This study is from Japan, but I'm pretty sure there are others too. For the USA or EU, I can

While reading it, I was always thinking about confounding factors. This study is from Japan, but I'm pretty sure there are others too. For the USA or EU, I can imagine many confounding factors. Let's pick one example: income. Wealthy families can hire a nanny. Even if the nanny lets the child watch something on screen for the same amount of time as an unattended child, it's different as the nanny can choose more appropriate videos or explain some context.