What about the evidence of earlier discharge at the same hospital? It wasn't clear how carefully that was evaluated, but surely that would nullify wealth, population density etc since the only difference was the room that people were put in.
First, just to be clear, issue at hand is the title, which states trees CAUSE calmness which CAUSES health.
All the research shows is there's a link between trees and health; aka there's a correlation, not causation.
Best example I'm aware of of "confounding factor" is that there's an obvious correlation between an increase in people drowning in pools and ice cream consumption rates. Eating ice cream does not cause drowning, nor do drownings cause people to eat ice cream. Hot weather causes people to swim more, which increases the odds someone will drown.
All I'm saying is this research does not show causation or look for confounding factors, which might show the correlation itself is a false positive.
This is why studying marginal effects -- like when the Emerald Ash Borer invaded county-by-county and caused a discrete time-series event that you can analyze before-and-after, as mentioned in the article -- is so critical.
