Film and Certification research.

List of 10 horror films and their certificates;

• The Exorcist - 18
• Saw - 18
• The Shining - 15 
• Dawn of the Dead - 18
• Night of the living Dead - 15
• Devil - 15
• Psycho - 15
• The Uninvited - 15
• Halloween - 18
• Paranormal Activity -15

From this, I conclude that 15 is the most common certificate.