there are differences between normal and natural birth. what has become normal in our society is not natural.
Thats just about it.

A normal birth to me is a vaginal birth without the intervention of any kind - as the OP has asked about "normal birth" and not natural birth.

The normal births that I see are those that progress normally with decent and dilation progressing as it should.

Everything else is a variation of normal.

I think as time goes on and women become more educated and more active in their own health care, variations to normal become normal. Things like posterior labours, VBAC's, epidurals etc.

So I guess its hard to pinpoint normal birth. It really all depends on your HCP, your own beliefs, your place of birth and the beliefs of those supporting you.

As for the article, I'm suprised it was even published given the lack of participants.