“Focal plane” (Survivor bias)


I sometimes hear tales about a childhood that consisted of unsupervised swimming, heavy whiskey drinking, riding in the back of a pickup truck, and living with asbestos and lead paint. This penalty-free daring continued in adulthood with helmetless motorcycle rides, and we’ve all heard from the 95-year-old great aunt who boasted about smoking a pack a day for three-quarters of a century. The insinuation is that none of these are a big deal since the person made it through unscathed.

But these are instances of survivor bias. Those who died doing those things are not here to tell us about it. There are also those who suffered nonlethal harm as a result. Dismissing those occurrences, especially when they are the norm, is the epitome of survivor bias, which relies on anecdotes over data. A person committing survivor bias focuses on entities that have made it through some selection, ordeal, or process, and assumes that represents the whole.

A person may say, “I wasn’t vaccinated and I’ve stayed healthy, so they’re unnecessary.” Or, “I have voraciously consumed ribs, eggs, and whole milk all my life and have below-average cholesterol. Those doctors and scientists don’t know what they’re talking about.” These conclusions take a sample size of one and apply it to the entire populace. They may even entail dismissing decades of properly-done research and empirical evidence. 

Moreover, even if a person turned out OK with a childhood of frequent smoking, drinking, and spankings, maybe he or she would have been even better without them. It might be more accurate to say that they turned out fine in SPITE of those things.

Then there’s the matter of where the ‘fine’ threshold lies. Is it merely maintaining mediocre employment while marrying, having children, and avoiding prison? I’ve heard persons who have managed not much more than this proclaim that they’ve “done OK.”

With regard to spanking, consistent research shows an increase in likely negative outcomes for those who are subjected to it. Unilaterally declaring that one has achieved an arbitrary benchmark of “doing OK” to assert that spanking is harmless is to dismiss sizable evidence to the contrary.

Another way of how it might work. One may say, “Vegans are so annoying, they always have to mention what they don’t eat.” But you are only hearing from vegans who tell you their dietary choices, not from those who refrain from doing so.

Survivor bias is also a regular feature of religion. In an interview with Larry King, Billy Graham cited a women who avoided a fatal crane plash through a series of delays that at the time seemed annoying but proved serendipitous. When Graham credited this to God, King pointed out there were scores of others who DID get on the plane.

Speaking of manned flights, the survivor bias term took root in World War II when Navy researchers studied damaged aircraft returning from missions. Their consequent suggestion was to add armor to the most-afflicted areas.  However, statistician Abraham Wald noted that researchers were only examining fighter planes that had made it back, meaning they could survive heavy hits to the affected areas. The better idea, he said, would be to galvanize parts of the plane that showed little to no damage since that is probably where planes that were shot down had gotten struck.  

Let’s look at some other examples. A study showed that cats which fell from less than six stories paradoxically had greater injuries than those who fall from six stories or higher. The initial suspicion was that the falling felines reached terminal velocity after righting themselves at five stories, after which they relaxed, leading to less severe injuries. However, a Straight Dope column suggested this was probably survivor bias since few dead cats would be brought to the veterinarian. Most of those who fell from six stories or more we likely killed on impact.

Now onto the plant world. Lianas are parasites that feast on trees, and the hosts of these unwelcome guests were seen to mostly be slow-growing and shade-tolerant. This led to a belief that lianas have stronger negative effects on these tree types. But further research showed liana infestation is actually more detrimental to trees that are light-demanding and fast-growing. So much more damaging, in fact, that it usually wipes them out, meaning researchers are less likely to find them.

The trees died, while failing mutual funds have a more figurative death. They are shuttered or absorbed into another fund. This means an investment company can accurately claim to be offering a better opportunity than what its track record would suggest since the mutual funds they are advertising are succeeding while those that failed have gone away.

During my time in Germany, I gazed in awe at the beautiful baroque architecture. But while it was amazing craftsmanship, that doesn’t mean J.S. Bach was surrounded only by stunning building designs in his time. It means those were the ones that survived because they were so amazing while the ugly ones were torn down.  

Likewise, most of us love stories with heroines like Barbara Corcoran, the Shark who turned a $1,000 loan into multibillion dollar business empire. But the idea that anyone with the right grit and inspiration can likewise become a successful entrepreneur, author, actor, soccer player, or inventor is survivor bias because we never see the failures. Nirvana and Apple both started in a garage, but there are many more bands and businesses who never left it.



One thought on ““Focal plane” (Survivor bias)

  1. My wife fell out of a pickup truck damaged her legs so bad she spent eleventh and twelfth grade on a wheelchair and crutches, she still has knee problems today, over thirty years later.
    Me, I still eat pork (and eggs) and won’t get a vaccine – even if my life depended on it… just saying

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