You should be able to find the mean between OP and a fixed craniofacial line such as Frankfurt horizontal, then mean between MP and Frankfurt horizontal, then figure out the difference. If using Arnett's analysis you'd use the True Vertical Line instead of a fixed line. I can do some digging at home if you still can't find your answer.

The relationship between OP and MP is less meaningful because one or both could be off of the desired and can be modified with jaw surgery and/or other cosmetic surgeries. It's better to analyze a changeable variable versus a fixed measurement.

edit: My document describing Arnett's analysis doesn't show an assessment of the mandibular plane at all. But I've attached a screen snip of the Down's analysis from a comprehensive analysis I had done at CephX.com. You'll see MP (GoGn) to FH mean is 28 degrees, OP to FH mean is 9.3 degrees, so the mean MP is 18.7 degrees steeper than the mean OP. I have no idea how many subjects were studied for this analysis, nor do I know if the subjects were considered normal or ideal or what. I believe it is an older study.

edit2: Wikipedia is showing a different mandibular plane angle for this analysis, see second attachment. Not sure which is correct.

"You should be able to find the mean between OP and a fixed craniofacial line such as Frankfurt horizontal, then mean between MP and Frankfurt horizontal, then figure out the difference."

I thought of the same but I'm not sure if this is legit and whats up then with the SD...

"If using Arnett's analysis you'd use the True Vertical Line instead of a fixed line. I can do some digging at home if you still can't find your answer."

How is the True Vertical Line defined and what is a fixed line?

"The relationship between OP and MP is less meaningful because one or both could be off of the desired and can be modified with jaw surgery and/or other cosmetic surgeries. It's better to analyze a changeable variable versus a fixed measurement."

I do not really understand this. Can you explain that for me?

"edit: My document describing Arnett's analysis doesn't show an assessment of the mandibular plane at all. But I've attached a screen snip of the Down's analysis from a comprehensive analysis I had done at CephX.com. You'll see MP (GoGn) to FH mean is 28 degrees, OP to FH mean is 9.3 degrees, so the mean MP is 18.7 degrees steeper than the mean OP. I have no idea how many subjects were studied for this analysis, nor do I know if the subjects were considered normal or ideal or what. I believe it is an older study.

edit2: Wikipedia is showing a different mandibular plane angle for this analysis, see second attachment. Not sure which is correct.

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These differences are huge! I think these two studies are not statistical significant or there were studied different ethnics, ages or gender. Often I see a Value with 25° +-5°.

Thank you for your help. I try to figure out if a ChinWing is sufficient or a double jaw surgery is necessary for my aesthetic problem...