“…To illustrate, in the 1-2 months following the September 11 terrorist attacks, Schlenger et al (2002), using the PTSD Checklist (PCL; Weathers, Litz, Herman, Huska, & Keane, 1993) as an assessment instrument, estimated that 11.2% of New York City-area residents met criteria for probable PTSD. This estimate was 50% higher than the 7.5% prevalence of PTSD estimated by Galea et al (2002) in a sample of Manhattan residents using the National Women's Study PTSD module (NWS-PTSD; Kilpatrick, Resnick, Saunders, & Best, 1989) as an assessment instrument. Applying each of these prevalences to the population of over 8 million New York City residents, one would estimate that approximately 900,000 (Schlenger et al) versus 600,000 (Galea et al) persons met criteria for probable PTSD, a difference of 300,000 people.…”