[driggs]

Thank you for pointing out that insightful detail!

The control group was 50% of the population who didn't get an invitation.

The experimental group was 50% of the population who got an invitation for colonoscopy, but turned out to be subdivided into ~40% "health conscious" who actually followed up on the procedure, and ~60% who ignored it.

Presumably there's a corresponding ~40% "health conscious" component of the control group, but this experiment had no method for identifying them.

If the study only looked at that ~40% subset of the experimental group, as opposed to the entire group who received invitations, then they could no longer compare them to the control group.