> Every tenth man in a group was executed by members of his cohort

That is a 10% reduction. Killing nine out of ten would be a 90% reduction but 'in the classical sense' it would also mean that the lucky guy would have to kill the 9 remaining people.

Why did you put 'in the traditional sense' in quotes, as if you were quoting the parent poster, who clearly wrote "in the classical sense"?

My mistake, I wrote it from memory instead of copying or looking at the parent post. Edited and fixed.

>Decimation (Latin: decimatio; decem = "ten") was a form of Roman military discipline in which every tenth man in a group was executed by members of his cohort. [...] The word decimation is derived from Latin meaning "removal of a tenth".[

Reducing a military force by 90% might not exactly improve its effectiveness...

Reduce by 10% or reduce to 90% of it's previous value