The significance is obvious: People in the US are getting healthier, by a significant metric. That doesn't matter? The US is a relativley well-defined group, sharing many inputs and consuming many of the same resources, including the same national health care resources for research, care, regulation, etc.
> There are many different populations in the USA.
Are you saying only your 'population' matters to you?
What do you mean by it exactly? There are lots of populations everywhere, and every population can be broken down into more populations. Any aggregate number won't describe you as an individual, even if it's a number for your own family.
Is this just a repeat of the old racial trope here?: https://news.ycombinator.com/item?id=46843222
no, i think you're imagining a lot of things. i'm just saying it is very coarse metric by which to understand anything at all. But, I'm not in any way educated in this metric so i'm open to anyone telling me how it is useful, like I asked initially.