This post first appeared on Lernabit.com. Lernabit helps you remember everything you learn about any topic.
One of the most common fallacies people commit is one called the anecdotal fallacy. The anecdotal fallacy is when someone bases an argument on anecdotal evidence. So the natural next question is: What is anecdotal evidence?
Anecdotal evidence is evidence based solely on the personal experience of one person or a small number of people. Such evidence cannot be used to make general statements that apply to everyone or every circumstance. Here is an example argument that demonstrates this:
Flying is a dangerous way to travel. I know because I was in a plane crash. We will much safer if we drive.
This argument commits the anecdotal fallacy. By using their own personal experience in a plane crash as evidence, the arguer is concluding that flying is more dangerous than driving. But if you were to look at statistical evidence, you would find that flying is actually a lot safer than driving.
The anecdotal fallacy occurs frequently in discussions about the supposed link between vaccines and autism. There are many stories about children receiving vaccines and being diagnosed with autism shortly afterward. This has led some people to believe that the vaccines are the cause of the autism. But by statistically analyzing large datasets, it is clear that there is absolutely no link between vaccines and autism.
The perceived link between vaccines and autism demonstrates the hazards of relying on anecdotal evidence. The reason anecdotal evidence is so unreliable is because often the only voices you hear are the most vocal ones. In any given dataset, most of the data is more or less average, but anecdotal evidence can give the impression that there is more variation than there actually is. To make a generalized claim about vaccines and autism — or any dataset for that matter — we need to look at the dataset using statistically valid techniques; we can’t just look at a small but vocal minority.
The main point here is that you simply cannot make broad statements based only on the firsthand accounts of one or a handful of people. If you do that, you are committing the anecdotal fallacy.