Seth Roberts, a professor emeritus of psychology from UC Berkeley and a self-experimenter, discusses three popular sayings about data.
“Absence of evidence is not evidence of absence.” Øyhus explains why this is wrong. That such an Orwellian saying is popular in discussions of data suggests there are many ways we push away inconvenient data.
“Correlation does not equal causation.” In practice, this is used to mean that correlation is not evidence for causation. At UC Berkeley, a job candidate for a faculty position in psychology said this to me. I said, “Isn’t zero correlation evidence against causation?” She looked puzzled.
“The plural of anecdote is not data.” How dare you try to learn from stories you are told or what you yourself observe!
Orwell was right. People use these sayings — especially #1 and #3 — to push away data that contradicts this or that approved view of the world. Without any data at all, the world would be simpler: We would simply believe what authorities tell us. Data complicates things. These sayings help those who say them ignore data, thus restoring comforting certainty.