Several complaints recently have been sent to the Gotham City police department regarding the increasing incidence of congestion on the city's streets. The complaints attribute the cause of these traffic tie-ups to a lack of synchronization of the traffic lights. The lights are controlled by a main computer system, and adjusting this program is costly. Therefore, the controllers are reluctant to change the situation unless a clear need is shown.
During the past year, the police department has collected data at 1,000 intersections. The data were compiled on a monthly basis as shown below:
Month Congestion Incidence
January 14
February 18
March 14
April 12
May 16
June 8
July 19
August 12
September 14
October 7
November 10
December 18

a. Construct a p-chart based on the above data.