Photo: Opo TerserFrom a reader named Mike Friedman:
I realized this morning that my daily behavior has been modified by a data point of one. I thought it would be interesting to you. Oh, and apologies in advance for the pun:
For the most part, my morning routine is the same every day. I shower, towel off, comb my hair, yada yada yada. A few weeks ago, while toweling off, I noticed a rather large spider on my towel. Being a little squeamish around spiders, I quickly threw down the towel and disposed of the intruding arachnid. I forgot about the “insect-dent” and went about my day.
Cut to this morning. After getting out of the shower and grabbing my towel, I experienced a moment of self-awareness and realized I was subconsciously checking my towel for spiders before applying it to my face. I realized in that moment of self-awareness that my behavior had been influenced by a single data point.
Consider the numbers: At 35 years old, I figure I have conservatively showered well over 12,500 times in my life, and I have only found one spider in my towel. And yet, for the last three weeks or so, I have been checking my towel every time I shower.
It got me thinking: How often do we allow our behavior to be influenced by single data points. Are there any positive examples?
Mike’s e-mail appealed to me because it touches on a lot of things that have been discussed here over the years, including recency bias, black swans, and the strange case of Baby Emily, whom we wrote about here:
In the early 1980′s, a group of psychologists and linguists banded together to write Narratives From the Crib, a study of how children acquire linguistic skills. Narratives was built around the speech patterns of one child, a 2-year-old girl. Her parents had noticed that she often talked to herself in the crib after they said good night and left her room. They were curious to know what she was saying, so they began to record her chatter. They turned on the tape recorder while they were tucking her in and then left it running.
Eventually they gave the tapes to a psychologist friend, who shared it with her colleagues. The big surprise to these experts was that the girl’s speech was far more sophisticated when she was alone than when she was speaking with her parents. This finding, as Malcolm Gladwell would later write in The Tipping Point, “was critical in changing the views of many child experts.”
The 2-year-old girl in question was referred to as Baby Emily. Her full name? Emily Oster. In retrospect, it would appear that Narratives From the Crib suffers what researchers call an “n of 1″ problem, with “n” representing the size of the sample set — a problem that is gravely exacerbated when the one subject turns out to be … well, a good bit brighter than average. Studying how children learn to talk by observing Baby Emily may be a bit like studying how children learn to play golf by studying Tiger Woods.
and 
. You can also watch it on 
Anecdotal evidence is not a scientific no-no — it is used all the time in the creation of theories — however when testing those theories, one needs more systematic data collection.
And with respect to looking at a towel — you would have been stupid to not check the next day — maybe there had been an infestation. And given the low cost of checking, it might be wise for you to check for another three weeks before going back to not checking your towel.
As a psychologist, I’d wonder if you’d do the same amount of checking if you’d found a cricket in your towel instead.
I see no scientific problem.
Verification and falsification are very different in how much evidence they require.
N=1 is sufficient evidence to disprove the hypothesis of spiderfreeness.
When you start looking, you find this all the time. The greater the trauma, the more visceral and automatic the reaction. I hit a car that I didn’t know was in my driveway while backing out of my garage about 8 years ago. I now automatically look before backing out and my stomach does flip-flops if I happen to forget and the realize it. I once slipped on some ice and fell and hit my head about 6 years ago. I have to force myself to walk on ice at anything other than flat-footed and slow. And it is all automatic.
A single data point can also be helpful from the perspective of Bayesian updating. While most of us regard frequentist observations as often more “scientific,” there is active debate about the value of Bayesian inference. Bayesian approaches do *not* preclude additional analysis with subsequent probabilities; if this had been applied to the Oster case, for example, the prior may have shifted again subsequently.
What passes for “research” in the field of education these days is rife with n of 1 problems.
Interesting because my wife once found a spider in her shoe as an adolescent. To this day, probably 15 years later, she still shakes out her shoes before putting them on. I sometimes tease her about it, but she will probably always do it.
There’s nothing irrational about checking the towel for spiders.
It makes sense to change one’s behavior after only one observation if you have reason to believe that the occurrence of the observed outcome is positively correlated with its re-occurrence. Here, as the previous commenter noted, there is the possibility of an infestation. Observing one spider should lead you to revise upward your estimate of the probability of observing future spiders.
Although you’ve showered over 12,500 times in your life, really you are only looking at the times you showered in your current location. I dare say that if you shower elsewhere, you won’t check and if/when you move you won’t think to check (of course, now that it was talked about you might).