Project Blue Book Special Report No. 14 was their massive statistical analysis of Blue Book cases to date, some 3200 by the time the report was completed in 1954, after Ruppelt had left Blue Book. Even today, it represents the largest such study ever undertaken. Battelle employed four scientific analysts, who sought to divide cases into "knowns", "unknowns", and a third category of "insufficient information." They also broke down knowns and unknowns into four categories of quality, from excellent to poor. E.g., cases deemed excellent might typically involve experienced witnesses such as airline pilots or trained military personnel, multiple witnesses, corroborating evidence such as radar contact or photographs, etc. In order for a case to be deemed a "known", only two analysts had to independently agree on a solution. However, for a case to be called an "unknown", all four analysts had to agree. Thus the criterion for an "unknown" was quite stringent.