Read on Twitter
Country-level correlation studies like the "lettuce will kill you" preprint provide epidemiologists with a good laugh-cry, & I want to be sure everyone understands why.
So why are diet/disease correlations (AKA ecologic studies) less than useful for deriving causality? 1/
So why are diet/disease correlations (AKA ecologic studies) less than useful for deriving causality? 1/
The Y-axis represents country-level disease prevalence or rates. Actually can be useful in characterizing between-country differences. Wow Belgium is concerning!
2/
2/
The X-axis gives the country-level average cabbage intake. Estimates are often based on food disappearance data or nat& #39;l surveys. If you& #39;re into that sorta thing, like the variability of cabbage intake worldwide, then how cool is this – Romania and Latvia for the win!
But the correlation between deaths and cabbage -- plotted as the country-level dots in the graph -- this is where it gets bad. Here is where were have:
FATAL FLAW #1:
3/
FATAL FLAW #1:
3/
So many reasons COVID19 deaths (or insert any outcome) differ between countries: GDP, public health policies, accuracy/transparency reporting, geography, health care access, on and on. Similarly, diet differs strongly by culture, GDP, health care, geography, lifestyle, etc.
4/
4/
Sound like confounding? Because it is! Adjusting for country-level factors is hardly sufficient when data are sparse or non-existent, making this virtually UNADJUSTABLE. Crude! Confounded!
5/
5/
Fatal flaw #2: Ecologic fallacy. Country level data are attributed to person-level outcomes; whether the COVID19 deaths occurred among Cabbage Avoiders is totally unknown severely hindering causality.
6/
6/
So where does that leave us? What knowledge have we gleaned?
Nowhere. None.
We can determine nothing from these data on the causal relationship between cabbage intake and death from COVID19. Remember this in a few weeks when the next ecologic study makes its rounds!
end/
Nowhere. None.
We can determine nothing from these data on the causal relationship between cabbage intake and death from COVID19. Remember this in a few weeks when the next ecologic study makes its rounds!
end/
