As is so often the case, the answer is money.

Big Tobacco's 'new errand boy'

From the researchers themselves!

This argument is fallacious, I believe, because there are many things that changed from one cohort to the next. The introduction of cigarette filters is only one such change. To assume that any observed changes in cancer risk from one cohort to the next must be attributable to one particular change - the filter - is not appropriate, in my view.

But what is the clinical meaning of this effect? In order to find that out, one needs to conduct studies with longer-term end points.

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