What you're saying is not totally correct. If you look only at patients that did get a colonoscopy, you cannot be sure that it is the colonoscopy that helps, or

What you're saying is not totally correct. If you look only at patients that did get a colonoscopy, you cannot be sure that it is the colonoscopy that helps, or another variable (those people might monitor their health closer). Your assignment is not random any more so you cannot make any causal statement.