medical measurements

A lot of our medical metrics are really terrible.  For example, cancer interventions are generally evaluated by n-year survival rate.  The problem is that the count starts at diagnosis, so you can raise your “survival rate” just by catching it earlier, by, say, widespread testing of people with no symptoms.  And in doing so you’ll catch a bunch of cases that get the same name (such as breast cancer) but would never have been caught by a symptom-driven search because they were never going to cause problems.  And this metric heplfully ignores any of the costs of testing, which can include cancer.  This is one reason the US has a higher cancer survival than Europe- we test much more aggressively.

Or take infant mortality, which is defined as [deaths shortly after a live birth]/[total number of live births]. This measure actually has two problems, if you’re trying to decide where you should have your baby.  One is that while Europe’s infant mortality rate is much lower than the US, the survival rate for a birth during any given week of pregnancy is higher in the US.  Europe’s advantage is that it has fewer premature births.  We don’t know why.  It’s certainly possible that this is another example of the USA’s sympathy-based aid distribution, which drives us to spend ungodly amounts of money on rare but high profile cases and neglect basic care that everyone would benefit from (like pre-natal care).  Or non-medical but still government-spending-driven interventions, like financial aid.  Or government-controlled-but-non-spending-policies, like pesticide usage or maternity leave.  Or non-government factors, like cultural norms and likelihood of close social bonds.  
 
Additionally, the definition of live birth is surprisingly malleable.  The US is much more into heroic intervention/batshit crazy belief that a 20 week old fetus is a person/tax deductions for babies that live for 12 hours, and events that would have been scored as miscarriages or stillbirths in Europe are counted as live births followed by rapid death here.   Theoretically there are statistical models that could give you comparable numbers for cancer survival, and backing out the effect of more premature births in the US is trivial, but there are no numbers that can tell us the magnitude of this discrepancy.
 
So if you’re an individual deciding where to live based on infant mortality, where do you go?  If G-d gives you a signed piece of paper saying you will go into labor at week N, you go to the US, no matter what N is.  Without that certainty?  My best guess is that sufficient money will buy you the advantages of Europe while living in the US, but the reverse is not true.  So this is another case where the rich are better off in the US and the poor in Europe.
 
So much comes down to what the number is used for. The fact that the infant mortality metric is driven by so many things beyond the medical system in front of you during birth makes it almost useless for individuals choosing where to give birth*.  But all those external factors actually make it more useful for large organizations trying to evaluate the health of a country as whole.  
 
The same thing is true of BMI.  As a quick and dirty metric to evaluate the change in a country’s average fat % over time, it’s not terrible.  It diminishes the risk of scoring an increase in average height- almost always a sign of improved health- as negative.  If your current population is of the same genetic stock as the previous, individual variation in ideal body composition will come out in the wash.  BF% is of course a much better metric, but it’s a billion times harder to measure, and given a fixed amount of money for a study, it’s entirely possible the wider sampling allowed by the cheaper metric leads to more informative results.  But BMI is completely and utterly <i>useless</i> as an assessment of an individuals health.  Genetic and epigenetic variation is simply too high.  When you are an individual (or their doctor), the time to measure BF%, or actual metrics of health like activity level and blood pressure, will always be worth it.
 
Further reading: