[oldgradstudent]

Virgin olive oil is more expensive.

The most important, and well known confounding factor in observational health research is socioeconomic status - rich people live longer than poor people.

This research attempted to control for it in a limited fashion, but that's not nearly enough.

[DoingIsLearning]

The study was in Spain were both the quality of olive oil is very high and the retail cost fairly low comparatively with the rest if the world.

Also many arguably poor farmers will grow their own olive trees and distribute the oil yield among their wider family.

So perhaps not so case closed as you say.

[moonchrome]

If that was true why would anyone use refined oil ?

[tluyben2]

You will have to live in Spain and know people to get high quality cheap oil. I have my own olive trees and we bring our olives to a small cooperative press; in that community the oil costs next to nothing, however, when it’s exported (and most here is for local production and is sold locally only), it is priced to what you find in the shop.

[moonchrome]

But isn't this study done on people from Spain?

If Spain is anything like Croatia you are not really poor if you own olive trees - you're probably middle class. Poor people sell that kind of inheritance very fast, and also buy cheap supermarket olive oil.

[tluyben2]

Land with olive trees is really not worth selling where I am unless it’s huge (and even then; people have 100000s m2 and no one is interested in it; you have to maintain it because of fire hazard and you cannot build on it). If it has a (legal) house on it then, depending on the state (if it’s inherited it will be almost or completely classed as a ruin most likely as it wasn’t kept up properly), it’s worth more, but still not a jackpot.

If you are poor and want to live here you are better of putting a caravan on your land than selling it for peanuts and not being able to do much of anything with that money (maybe 1-2 months rent). At least working the land will pay you something and you can trade olives for other goods (many of my neighbours live like that).

Most land / ruins won’t get sold at all; people move away to the cities for jobs and just forget about it, or, family feud and they cannot agree; the result is the same. Usually the neighbour just uses it as their own ‘until they come back’ (which often is never).

[eloff]

Or simply that people who care more about their health are more likely to purchase extra virgin olive oil. Unsurprisingly, those people might have lower mortality. These kinds of studies are often useless. The authors try to control for some confounding factors, but reality is just too messy.

[brutusborn]

This is so common in these kinds of studies that I tend to ignore them. I saw a similar one recently that linked certain types of meat and dairy consumption to longevity but didn't control for wealth. The real problem isn't diet but being poor.

[oldgradstudent]

Yeah, it's almost looks they're written by statistics professors trying to illustrate the concept of confounders.

[dzdt]

This.

Looking at this kind of research is a kind of game: statistically most such research is wrong [1], so the game is to spot the mistake as to why the research is wrong.

I think you are winning this round!

[1] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1182327/

[oldgradstudent]

In this field of observational epidemiology, "most" is an understatement.