North Korea has never published any number on military personnel. Yet, due to its belligerent attitude, it could be important for South Korea and others to estimate how big is the (non-nuclear) North Korean threat. How can one get an estimate? Just ask Moon, the forensic economist.
As he writes on Vox, "North Korea may have accidentally published its number of military personnel. A close examination of the census numbers reveals an anomaly. The population overall is greater than the sum of the populations by administrative district". This is because citizen registration has to be returned to the public security office when the citizen enlists in the Army. This implies that the (700,000) discrepancy consists in part of those enlisted in the army. What's more, when looking at the sex ratio of the census numbers (figure below), one can see a sharp dent between ages 16 and 26. This drop coincides exactly with the end of compulsory-education, when some join the armed forces, and is entirely due to a decline in the number of men. It is likely that these "missing men" are members of the armed forces.