Consider the following possible responses to the three research questions:
A man on the news got mercury poisoning from eating swordfish, so the average mercury concentration in swordfish must be dangerously high.
I met two students who took more than 7 years to graduate from Lancaster, so it must take longer to graduate at Lancaster than at many other Universities.
My friend’s dad had a heart attack and died after they gave him a new heart disease drug, so the drug must not work.
Each conclusion is based on data. However, there are two problems. First, the data only represent one or two cases. Second, and more importantly, it is unclear whether these cases are actually representative of the population. Data collected in this haphazard fashion are called anecdotal evidence.
Anecdotal evidence
Be careful of data collected in a haphazard fashion. Such evidence may be true and verifiable, but
it may only represent extraordinary cases.
Anecdotal evidence typically is composed of unusual cases that we recall based on their striking characteristics. For instance, we are more likely to remember the two people we met who took 7 years to graduate than the six others who graduated in three years. Instead of looking at the most unusual cases, we should examine a sample of many cases that represent the population.