Final answer:
The figure stating that 20% of troops used heroin during the Vietnam War would likely have been derived from a combination of methods including self-reporting surveys, medical records, and government statistics, amidst a broader context of unreliable and manipulated data during the war.
Step-by-step explanation:
The statistic that 20% of troops used heroin during the Vietnam War was obtained through various methods, including self-reporting surveys, analysis of medical records, and other government data. While not explicitly stated in the documentation provided, it is common practice in such historical contexts to use a combination of methods to establish such figures, given the challenges of collecting data in wartime conditions.
During the Vietnam War, both official government statistics and firsthand accounts from troops highlighted discrepancies in the official narrative. Government reports on the war progress often conflicrated with the reality experienced by soldiers and reported by journalists on the ground - a phenomenon that became known as the credibility gap. Reports by military and media alike painted a complex picture of the situation, where data could have been manipulated or underreported, including aspects like drug use among troops.