There are differing opinions to how many victims the Zodiac may have condemned to an early grave, ranging from as little as five or less in his widely publicized canonical crimes, to as many as thirty-seven or more, claimed in his final confirmed correspondence on January 29th 1974. The canonical crimes began on December 20th 1968 with the murders of David Faraday and Betty Lou Jensen on a lonely turnout in Benicia, and ended with the brutal and callous execution of Paul Stine in Presidio Heights on October 11th 1969.
But can we use mathematics and statistical analysis of his running murder count to evaluate the voracity of the Zodiac Killer's claims. This will be an experimental examination, using the frequency of his alleged murders, to unlock the likelihood of whether he carried on murdering after October 1969, or he simply used statistics himself, and painted a picture of murder by numbers. The Zodiac Killer claimed to have murdered seven people between December 20th 1968 and October 11th 1969 (stated on the Dripping Pen card). That is a span of 295 days, or one victim every 42.14 days. The following will have to use the postmark on communications as the time period - so will be approximate.
His rest periods from Lake Herman Road to Presidio Heights had considerably shortened between attacks, however, his alleged victim count didn't maintain this pace up to the arrival of the Exorcist letter. From the March 13th 1971 Los Angeles letter to the January 29th 1974 Exorcist letter, he would add a further 20 victims (again disregarding the + symbol for now). If the killer was indisposed for a significant period of this communication 'absence', was he able to create a realistic frequency to his running victim count when he resumed writing. This period of 1,053 days indicated a victim count frequency of 52.65 days - closely aligned with his 47.82 days from December 1968 to March 1971.
March 13th 1971 possibly marked the last day the Zodiac Killer used the pseudonym under which he was killing. He would never use it again in any future confirmed correspondence, including four letters received in 1974. The addition sign was likely signifying the crimes he was claiming down south, including the murder of Cheri Jo Bates in Riverside on October 30th 1966, when he stated "This is the Zodiac speaking Like I have allways said, I am crack proof. If the Blue Meannies are evere going to catch me, they had best get off their fat asses + do something. Because the longer they fiddle + fart around, the more slaves I will collect for my after life. I do have to give them credit for stumbling across my riverside activity, but they are only finding the easy ones, there are a hell of a lot more down there. The reason I'm writing to the Times is this, They don't bury me on the back pages like some of the others".
After his so-called admission of previous crimes, that may have included the Domingos and Edwards murders in Santa Barbara on June 4th 1963, unlikely to have been committed under the Zodiac umbrella, it may have been the final curtain for the Zodiac Killer. His return just under three years later had dropped the "This is the Zodiac Speaking" opener, and his crosshairs were notably absent. He was reduced to Me- 37 SFPD- 0. But his change may have come a lot sooner than that.
The Zodiac maintained a statistically significant victim count throughout his five-year reign, that may have been more by design than actual reality - something that was closely aligned with the delivery of his communications. The author had throughout maintained a close correlation between his victim count and the correspondence he mailed. By the time his 'in tandem' communications of the Dripping Pen card and Bus Bomb letter had arrived, he had mailed, in total, seven letters, and was claiming 7 victims. By the time he had mailed the 13-Symbol cipher on April 20th 1970 (including the Fairfield correspondence), he had mailed 10 letters and was claiming 10 victims. When the Button letter arrived on June 26th 1970, both communication and victim count totaled 12. There was a slight difference shortly after that, but parity would be soon resumed by March 13th 1971, when the Los Angeles letter arrived - his seventeenth communication with 17 victims.
The Zodiac Killer appeared very structured in the frequency of his alleged victim count and the regularity of his communications, bound tightly to one another. But was it too structured, given his marked absence for nearly three years, thereby lending credence to the idea that it was all just a case of murder by numbers.