My understanding is that there is no directly comparable number in the ORs which actually properly compares the strengths of two units that is consistent throughout the war, for these reasons:

- PFD is not consistent in all returns

- Effectives is not usually given (and is not consistent)

- AP is consistent in definition, but for some Confederate army units their AP is basically "all men with the army who are not black" which causes problems.

To elaborate on that last one, imagine a Confederate force of 10,000 AP and a Union force of 10,000 AP.

The Union force has (rough numbers) 2,000 men doing logistics work, special and extra duty, and so has a calculated effectives of 8,000.

The Confederate force has (rough numbers) 2,400 men doing logistics work, but 1,400 of them are slaves or free blacks (who are thus not part of the AP), and so has a calculated effectives of 9,000 and a total number of men with the force of 11,400. So the Confederate force is actually larger, but if you looked at the AP it'd look the same size.