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 had claimed to have murdered seven people between December 20th 1968 and October 11th 1969. That is a span of 295 days, or one victim every 42.14 days. The following will have to use communication arrivals as the time period, so will be approximate.
That is 1,866 days, or one victim every 50.43 days. The next relevant time span is from December 20th 1968 to March 13th 1971 when the Zodiac Killer was claiming 17+ victims, before his communications ceased for nearly three years. Dispensing with the plus symbol for now, this gives a frequency of one victim every 47.82 days.
From the 'Los Angeles' letter of March 13th 1971 to the arrival of the 'Exorcist' letter on January 29th 1974 his communications to the newspapers were no more (omitting the 'Pines' card), leaving some to speculate he may have been abroad, or incarcerated in prison or a mental institution. If the Zodiac Killer was denied the opportunity of murder during this period, did he have the foresight to create a viable murder count during his absence, or if still committing murder, had his frequency slowed or increased.
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.
It is not likely the Zodiac failed to remember how many victims he had murdered during this relatively short period.
The use of 17+ to denote his victim count is also an extremely curious and rather meaningless total in regards to current crimes. If you had killed 30, 50 or possibly 100 victims and had lost count, you may write 50+ for example, but you certainly don't write a specific number such as 17 and bolt an addition sign to it, when it was only a few months since you were claiming 14.
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 7 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.