Bruce Catton wrote in his 1951 work "The Army of the Potomac" the following...

"The Rebels counted only the men who would actually be carring [weapons], and the Federals counted all who were "present for duty"- which meant that they included all the cooks, orderlies, train guards, ambulance details, and others and others who had non-combat assignments".

Federals forces combat strenghth could have been 20% too high in battle reports.

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David Upton