Bayesian probability basically says that “probability” is, to some degree, subjective. It’s your best guess for how likely something is. But to be Bayesian, your “best guess” must take the observable evidence into account. Updating your beliefs by looking at the outside world is called “Bayesian inference“. Your initial guess about the probability is called your “prior belief”, or just your “prior” for short. Your final guess, after you look at the evidence, is called your “posterior.” The observable evidence is what changes your prior into your posterior.
How much does the evidence change your belief? That depends on three things. It depends on A) how different the evidence is from your prior, B) how strong the evidence is, and C) how strong your prior is…
When those people keep broadcasting their priors to the world again and again after every new piece of evidence comes out, it gets very annoying. After every article comes out about a new solar technology breakthrough, or a new cost drop, they’ll just repeat “Solar will never be cost-competitive.” That is unhelpful and uninformative, since they’re just restating their priors over and over. Thus, it is annoying. Guys, we know what you think already.
English has no word for “the constant, repetitive reiteration of strong priors”. Yet it is a well-known phenomenon in the world of punditry, debate, and public affairs. On Twitter, we call it “derp”.
Which is to say, a policy commentator is “derpy” when his or her (usually his) prior assumptions about the world are so unwarrantedly strong that he is unswayable by evidence. Derpers have a faith-based approach to policy.I came to this derp definition via another post from Barro and also Krugman on why perceptions of inflation in the USA don't match the actual statistics:
Erickson’s rant is literally the definition of derp. He has a strong prior inclination to believe that inflation is high. As such, his view is not responsive to strong evidence to the contrary, such as BLS data showing very low inflation both overall and in the specific products that Erickson is whining about.The same perception is relevant to Australia where perceptions of the high "cost of living" don't match the statistical evidence. Yes, there have been some high profile price hikes in electricity and insurance but CPI and relevant indexes have remained benign. So how did the "cost of living" issue become elevated to such prominence? Derp!
Note: Stephen Koukoulas has written on this several times in recent years: Cost of Living?