In preparation for the visit of Pope John Paul II on Oct. 5, 1979 to Chicago, Illinois, a multi-departmental committee met for a number of months prior to the Pope's visit to prepare for possible mass casualties at the outdoor papal mass held in Grant Park. Data taken into consideration in preparation for this mass gathering included predictions of the number of people to attend the event, calculations by the National Safety Council of the possible number of victims and type of casualties, a review of the literature and the experience of others at outdoor events of the same scale.

On this basis, it was predicted that aproximately 3,000 to 3,500 casualties would occur simply as a result of a mass gathering of one million individuals. The actual total number of casualties was 400, with twenty being transported to the hospital and no deaths. On the basis of this experience a formula was described to predict the number of casualties based on total attendance figures.