Tag Archive: Data Skeptic


Coincidence City

(Submitted by Skepticality listener Christie Greene)

I have a challenge for you. I was on a plane from Denver, my home, to Nashville to visit a college friend. She and I were roommates at a college in Nashville.

I was born in TN and moved to CO at age 23. I was in the center seat on the plane with a man next to me. We did not speak and were caught up in our books/computers/earbuds.

As we were descending into Nashville, we were told that we had to divert due to a weather event. The atmosphere in the cabin changed to something more relaxed, as so often happens when a diversion occurs from what is expected. At this point, this fellow and I began a conversation. I will stress here that if we had landed, said conversation would have never taken place.

The guy was from a city further west from Denver and had made a connection there. He was, at it turns out, flying into Nashville as his final destination, as I was. As we spoke, he told me that he was attending a funeral in a town not too far from Nashville. When asked which town (remember, I am from West TN), he told me the funeral was to be in a tiny town called Selmer. Selmer is actually about a 2 hour drive from Nashville.

I turned to him, astonished. I have an aunt, uncle and cousins who have basically lived in Selmer their whole lives. Wow, what a coincidence! But it gets better.

As we talked, he mentioned that he would be taking the ashes of the deceased to be scattered at a lake nearby, about an hour’s drive from Selmer. When I asked where this would be, I was floored by his answer. The lake and town to which he would be traveling with the ashes was Savannah, TN and Yellow Creek, a dammed area of the Tennessee River.

I graduated high school at Central High School in Savannah in 1981 (i only lived in Savannah for 4 years, mind you) and my extended family owned a small vacation home on Yellow Creek.

Okay, Skepticality, what are the odds?


Below are the extended notes provided by statistician and podcaster Kyle Polich for use in Skepticality Episode 272.  Take a look and leave your comments below. Also, please be sure to listen to the podcast for our own hilarious commentary.

(Kyle studied computer science followed by artificial intelligence in grad school with a focus in probabilistic reasoning and planning. His general interests range from obvious areas like statistics, machine learning, data viz, and optimization to data provenance, data governance, econometrics, and metrology. He enjoys exploring the intersection of statistics and skepticism and sharing related insights with others including through his podcast Data Skeptic. Visit Kyle’s blog Data Skeptic, and give the podcast a listen.)

Christie’s new acquaintance from the flight happens to mention his destination is the town of Selmer, two hours drive from their landing city of Nashville. He goes on to reference two other small towns, also within two hours of Nashville, to which Christie also has a connection.

Determining just how crazy these odds might be requires an understanding of how connected we are as people. I, for example, live in Los Angeles, California. I know people who live in Santa Monica, Culver City, Hollywood, Monterey Park, La Habra, Studio City, Pacific Palisades… I don’t know anybody from Malibu… anyway, what percentage of towns within two hours drive of me do I have a connection to?

I wrote a program that looks up that list of cities for any input. I generated a list of cities near a few of my friend’s homes and I asked them to tell me which municipalities they had some connection to. From this, I could come up with the frequency that people I know have a connection to cities near them.

To my surprise, I got extremely varied results. Some people had a connection to as few as 5% of nearby cities, while my highest scoring participant claimed to be connected to 70% of nearby municipalities.

Given my wide variety of results, I want to turn the tables on you, the listener. Guess for yourself, what percentage of municipalities within two hours of your home do you have a connection to? 10%? 50%? Think about it, and come up with a percent. Once you’ve got it, imagine you have a coin. But this coin is a weighted trick coin which comes up heads as often as your percent. So if you have few connections to nearby cities, say 1%, then on average, only 1 toss out of 100 is expected to be heads. Hang on to your imaginary coin, we’re going to be flipping that in a minute.

As far as we know, the gentleman in our story called out three cities in a row that Christie had a connection to. This is the equivalent of getting three heads in a row on your imaginary coin. That being the case, we can apply some basic binomial probability to this situation.

If you are connected to only 1% of nearby cities, than your odds are exactly one in a million. But I think that’s extreme. Most people are connected to more cities than that, especially in areas they grew up in. I have a connection to 40% of the cities within 2 hours of where I grew up near Chicago, so for me, the odds of 3 hits in a row are exactly 6.4%. And for anyone connected to only 10% of nearby cities, the odds drop to 0.1%.

So the exact degree of craziness in these odds relies entirely on how connected we are to people in cities that are around us. The less connected we are, the more surprising. I think assuming people are connected to 10% of the places within 2 hours of them sounds conservative and reasonable, so by that frequency, the chances are a bit small at 0.1%, or one chance in a thousand.

A Classic Coincidence

(Submitted by Skepticality listener Ian Dodd)

In May of this year I attended a conference of humanist organizations in Atlanta, Georgia where I had a conversation with one of the local organizers. She told me she had a brother who was living in Hawaii but considering a move to Los Angeles, where I live, sometime later in the year and asked if she could pass my phone number on to him.

I had forgotten about the exchange until last week when I got a call from an unknown 808 area code number. The young man on the other end of the line explained who he was and how he had my phone number. We chatted briefly and I found out he and his wife had arrived in LA, they were looking for a place to rent and we made a date for lunch with a couple days later.

As we got to know each other over lunch, I learned that they knew nothing of the organization his sister and I are both affiliated with, so I told him how it was I came to meet her. Then they asked about my family and I explained that I had two children, one not much different in age from them, who had graduated from a small college in Minnesota in 2014 with a degree in Classics.

The young woman interjected, “Your daughter didn’t happen to go to Carleton College, did she?” Which, if you’re listening to this podcast, you can already guess what my answer was. Listeners should understand that Carleton is a college of 2,000 students in rural Minnesota. This young woman explained that her childhood best friend from growing up in Houston, TX had graduated from Carleton the year before in 2013, also in Classics, a department of about a dozen students.

I texted my daughter and my lunch companion texted her childhood friend to ask if they knew each other only to find out that the two of them had been study buddies through ancient Greek language for the 3 years they overlapped and are still close friends.

And by this last weekend, they had found a place to live: they will be renting from my wife and me starting in a couple weeks.

Seriously? The odds of this must be crazy!


Below are the extended notes provided by statistician and podcaster Kyle Polich for use in Skepticality Episode 264.  Take a look and leave your comments below. Also, please be sure to listen to the podcast for our own hilarious commentary.

(Kyle studied computer science followed by artificial intelligence in grad school with a focus in probabilistic reasoning and planning. His general interests range from obvious areas like statistics, machine learning, data viz, and optimization to data provenance, data governance, econometrics, and metrology. He enjoys exploring the intersection of statistics and skepticism and sharing related insights with others including through his podcast Data Skeptic. Visit Kyle’s blog Data Skeptic, and give the podcast a listen.)

So this story covers a series of seemingly unlikely events. Let’s try and break them down and isolate the parts that are not surprising from the parts that are eyebrow raising.

One of the important lessons here is around *conditional* probability. What is the probability that a person can play “Mary Had a Little Lamb” on the bassoon? Pretty low! What about the probability of that given the fact that they’re a professional bassoon player – very high!

To begin with, our listener is out of town chatting with a conference organizer who mentions her brother is moving to the listener’s city. There are almost 40k municipalities in the United States, so shall we say the odds of this are 1 in 40k or 0.0025%? Not quite.

Let’s consider the complement of this situation. Imagine you meet someone and proudly announce “I’m from Los Angeles”, to which they reply, “Cool! I have a good friend that lives in Gainsville, FL!” I mean, that’s nice, but I’m from LA. I think it’s fair to say our investigation only starts *conditioned* on the fact that a common city comes up in conversation.

Moving ahead to the part of the story in which the listener meets the relocating young brother and wife, and mentions having a daughter who attended a small college in Minnesota in 2014 with a degree in classics. The young women mentions having a close childhood friend who studied the same subject in a Minnesota school, and asks if they might perhaps have attended the same school and know each other. There are almost 200 colleges and universities in Minnesota. I’m not sure what qualifies as small, but if half of them are considered small, we can call those odds about a 1% chance.

Setting aside how many childhood friends the young woman had and how many universities they spread out into, maybe we call these odds 1 in 100 chance. That’s like betting on a specific number for rulet and winning. Unlikely, but not extraordinary.

But now we get into *conditional* probability. What are the odds that two students at a small school in a small department of about a dozen students know each other? I should hope pretty high!

So all in all, I find this one noteworthy, but not excessively surprising, and if I had to put a firm number on it, I’d say in the neighborhood of 1% likelihood.

The US Census tracks state to state movement.  Kyle put together a fun, interactive data visualization that allows people to select a state and see the percentage of people that leave that state and what other states they migrate to.

http://dataskeptic.com/tombc

(Submitted by Skepticality listener Andrea Monticue)

Last Sunday, I was driving and listening to the audiobook by Mira Grant, “Parasite.” The story takes place in the near future, and the characters live in the Bay Area of California. In the book, the main character and her sister decide to go shopping at “the big mall in San Bruno.”

Guess which parking lot I was pulling into when I heard that phrase? Yes, the “big mall in San Bruno,” California, otherwise known as Tanforan.

I go to that mall about once every couple of months. I’m not a fan of big malls, but there’s a Barnes & Noble there.

The only reason I’m listening to “Parasite” is because I enjoyed Grant’s zombie trilogy. 


Below are the extended notes provided by statistician and podcaster Kyle Polich for use in Skepticality Episode 260.  Take a look and leave your comments below. Also, please be sure to listen to the podcast for our own hilarious commentary.

(Kyle studied computer science followed by artificial intelligence in grad school with a focus in probabilistic reasoning and planning. His general interests range from obvious areas like statistics, machine learning, data viz, and optimization to data provenance, data governance, econometrics, and metrology. He enjoys exploring the intersection of statistics and skepticism and sharing related insights with others including through his podcast Data Skeptic. Visit Kyle’s blog Data Skeptic, and give the podcast a listen.)

I suppose the question here is: “What are the odds of arriving at a specific location just as a character from an audio book is arriving at the fictional version of the same location?” but actually, that might be the wrong question to ask.

This seems like a case of postdiction, also known as the hindsight bias, or put more simply, a case of remembering the hits and forgetting the misses.  Most people have had the experience of thinking of someone and then immediately getting a phone call or text message from them. I have to confess, that always does feel a little spooky, even to me. But in reality, if every time I thought of a friend, acquaintance, of loved one, they ended up calling me right away, I’d be endlessly annoyed with how many impromptu calls I’d have to take. Yet, the goose bumps that sometimes accompany these infrequent coincidences make them memorable.

There must be dozens, maybe hundreds of other malls in the bay area that the author could have chosen.  As of the time of recording, the Wikipedia page for San Bruno, CA lists five specific locations that are not parks or schools, one of them being – you guessed it – the Tanforan Mall. For me, this is enough to say that it’s not surprising that a story taking place in San Bruno might feature a scene at this location.

If you studiously compiled a list of actions taken by characters in Parasite, I suspect you’d be surprised to find how long this list is with only one memorable overlap to your own actions. So while precise likelihood is hard to establish here, I think this tale is a great reminder that experiencing a few seemingly odd coincidences every so often is really the norm, not the exception. Google Littlewood’s Law for further reading if you’re interested. And just to prove the point, I want to say congratulations to a certain listener who has recently taken a new job. I won’t say who, but if you’ve had a career change in the last 3 months, my congratulations go out specifically to you.