Article: For over 40 years, the FBI crime lab in Quantico believed lead in bullets had unique chemical signatures. It analyzed the bullets with the goal of matching them chemically, not only to a single batch of ammunition coming out of a factory, but also to a single box of bullets. The National Academy of Sciences conducted an 18-month independent review of comparative bullet-lead analysis. In 2003, its National Research Council published a report whose conclusions called into question 30 years of FBI testimony. It found the analytic model used by the FBI for interpreting results was deeply flawed, and the conclusion, that bullet fragments could be matched to a box of ammunition, was so overstated that it was misleading under the rules of evidence. One year later, the FBI decided to stop doing bullet lead analysis.

Question: How long was the National Academy of Sciences independent review?
18-month