This graph shows you the incidence of esophageal cancer across the various tea categories. Not too impressive right? In fact, the highest incidence is in the people who don’t drink tea very much – that’s the reference group in the adjusted analysis.

In other words, an unadjusted analysis suggests that drinking tea is protective.

OK, I thought, there must be a factor that is linked to esophageal cancer that makes you less likely to drink tea. What could it be?

I’ll spare you the sleuthing, except to say that it involved a series of emails with the primary authors.

The authors were nice enough to do some extra analyses and told me that this was due to stratification by region.

What this means is that there are areas in China where hot tea drinking is low, but esophageal cancer rates are high. But if you look within each area, you find that hot tea is associated with more esophageal cancer, even though the cancer rate in hot tea drinkers from Area A may be less than the cancer rate in warm tea drinkers in Area B, you only compare people to their Area compatriots.

You can see the effects of region-based stratification in this graph.