| Contributed by
Vijay Fafat
This amusing, if a bit farcical, little tale unfolds in a bar on a very rainy night, where Sam Hourani, a homicide detective, recounts to the storyteller how he thinks that a recent “accident” was “probably murder”.
That evening, a woman who had gone down to do laundry in her basement slipped and broke her neck, dying instantly. At that time, her husband had gone out to run some errands, and there were no signs of any foul play, like basement stairs being greased. And yet, Sam is convinced that her husband is statistically a murderer. Why? Because the husband is a statistician, an actuary who knows the statistical risks of all kinds of events – smoking habit, going out on a stormy night, a medical anomaly, etc. And he kept asking his wife to do all the “hazardous” things in life like going up and down the basement, not quitting smoking, perhaps taking random ganders outside the house, and so on, hoping that on event or another would get her. And ultimately, one did… So statistically, it is a probable murder which may never get proved…
A long excerpt should give summarize the story (which is not much longer than the piece below):
| (quoted from Probability Murder)
“Death by falls is the second greatest cause of death. The probability is...” Sam pulled a notebook from his breast pocket. “...is 56.4908 deaths per million per year.” He looked up. “That’s an aggregate figure, of course. For each trip up or down, it’s even smaller. But every time he made her do it, it was one more opportunity for a fall.”
“But,” I protested, “that hardly makes the poor felly a killer!”
“Not by itself. But he also insisted that he drive the car, which meant that she always sat in the right front seat, where the risk is greater. No passenger-side air bag, either. He took her swimming every week, though he never went in himself, and death by drowning is 14.9236 per million. He insisted she bathe, not shower. That’s 1.2439. He encouraged her to smoke. He’d broken the habit, but he told her it made her look sexy and discouraged her sporadic efforts to quit.” The lights blinked and a clap of thunder shook the windows, causing us all to flinch. Even Danny stirred and almost raised his head from the counter top. Detective Hourani smiled a little. “He would have found some excuse to send her on an errand tonight. Death in cataclysmic storms is 0.3506 and lightning is 0.2375 per million. “Well,” I said, “that may be abusive behavior, but how can you say he murdered her?”
Sam sipped a little more from his martini. “I told you. He’s a statistician;
works as an insurance actuary, and he knew every single one of those risks, medical
and casualty. Kept them in a notebook at home, which is where I got these...” He
waved his notebook. “It works this way, you see...” He flipped a page. “The
survival rate is one minus the risk. To find the chance of surviving all the risks, you
multiply the survival rates.”
“So if there are two things that can kill you,” I wondered aloud, “say, 10% of the time; then the chance of surviving both is point-nine times point-nine, or 81%.”
Sam nodded. “That’s it. “So, then,” said Himself. “This felly—” “—exposed his wife to as many risk factors as he could, every day. Sure thing, she’d probably survive any one of them; but she’d probably not survive all of them. That’d be 0.9999435092 times 0.9999850764 times 0.9999996494 and on and on. Multiple trips up and down the stairs with a laundry basket; multiple trips in the car; two or three packs a day of cigarettes ... Chances of long-term survival dropped exponentially. Oh, the guy killed her, all right. He was patient. He worked the numbers. He saw me reading his notebook and he smirked at me, the damned bastard. That’s when I knew. That smirk. He couldn’t just be clever. He had to know that I knew how clever he was.” I glowered at the wickedness of men. “D’ye think you’ll ever get him?” Sam drank the rest of his martini and placed the glass on the bar. He smiled coldly.
“Probably.”
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