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Confirmation bias can lead to wrong conclusions


I met Dr. David Zylneck through a mutual friend when he was in the final year of his pediatrics residency. Even as a trainee, he was heavily involved in international relief work and had made several trips to Southern Africa. Over time, Dr. Zylneck and I lost contact as our careers took us in different directions. Several years later, as I was about to move to a new location to start a new job, another friend, Joan, told me about a local pediatrician who had made several trips to Africa. I responded that I knew someone who fit that description. When Joan mentioned the name of the hospital where the pediatrician worked, I was pretty sure that was the hospital Dr. Zylneck was affiliated with. You can imagine my surprise when I asked for his first name, and she responded, “David.”

“Wow! That definitely sounds like someone I know. What’s his last name?”

Joan couldn’t recall the correct pronunciation of David’s last name but said the first three letters were “Zyl,” and the last two letters were “ck.” She also thought there was an “e” somewhere in the middle.

“Wow! That has to be David Zylneck! What a small world!”

When Joan offered to make an introduction after I had moved and started my new job, I jumped at the opportunity. When the day of the meeting finally arrived, I was surprised that I did not recognize the person in front of me.

“Hi, I’m David Zylwerck. I’m a retired pediatrician,” he said. After the introductions, Dr. Zylwerck told me that he had recently retired from the local hospital but that he was still involved in international relief work. I listened intently as he told me interesting stories of his trips to Northern Africa.

After that meeting, I couldn’t help wondering about the similarities between the two Davids. I mean, what were the chances? Same first name and very similar last names? Both with significant experience traveling to African countries, albeit different regions? Same specialty, same hospital, though one was retired?

Jumping to conclusions with limited information

This experience made me think about how easy it is to jump to wrong conclusions, even based on what appears to be “overwhelming” evidence. In the story above, what could have been interpreted as “overwhelming” was actually very limited evidence. Let’s review the information.

Similar names

Two people having the same first name isn’t that unusual, especially with a relatively common name like David. Add to that the similarity in the last names, and it’s easy to take this as “strong” evidence. But then, the information about the last names was incomplete. With the information about the letters in the last name, it was easy to assume that the remaining letters would be the same. As we have seen, though, the last names, though similar, were, in fact, different.

Travel history

Both Davids had a significant history of travel within Africa. That might have been convincing enough, until it was revealed that their travel experience was in different parts of Africa.

Work history

Both doctors were in the same specialty (pediatrics) and had worked in the same hospital, but one was retired and the other was not. Now that we have all this information, we can see that there are significant differences between Drs. Zylwerck and Zylneck, despite their similarities. If all the information had been available from the beginning, there would have been no reason to confuse them.

Confirmation bias

Confirmation bias is real. When we want something to be true, we tend to latch on to pieces of “evidence” that confirm what we already believe and ignore other pieces of information that contradict it. Imagine if Joan had told me that the David she was describing was retired? Since the David I had in mind was not yet of retirement age, I may have either assumed that she was mistaken or assumed that David Zylneck had retired prematurely. If she had told me that his trips to Africa had been to the northern part of the continent, even though I knew that David Zylneck had never been to any country in northern Africa, I could easily have assumed she was mistaken with regard to this “minor” detail, since the other details seemed to line up with my preconceived notion.

Clinical implications

It’s easy to assume we have all the answers when we can’t see the full picture. At the end of the day, we sometimes will have to make clinical decisions based on limited information. As clinicians, we need to be aware of our own biases (even when we think we don’t have any) and ensure that they are not preventing us from making the best decisions for our patients. We should also have the flexibility to acknowledge when there is a need to change course in light of new information.

Author’s note: This is a fictionalized account of a true story. While the names and exact details have been changed, the doctors described are real people, and the extent of similarity described (names and professional experience, etc.) is very real.

Olapeju Simoyan is an addiction medicine specialist.






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