Base rate and mean reversion structure

One decision making suggestion is to start with the base rate, to find a set of comparable circumstances and ask, ‘what typically happens?’

In investing it is to find the price of, say Amazon, and ask how often a company of those characteristics grew at an expected percentage. Rather than start with the idea that Amazon is a great company that does a bunch of things well and so on – we can ask, for companies like this how many have grown at 15% a year? Zero.

Another example is (large, but maybe all) construction. The “base rate” is not good, fast, and cheap but overpromised, over-budget, and underwhelming. You cannot compare this to that people protest. Boloney says Bent Flyvbjerg, the cost overruns and benefit shortfalls are so consistent they are comical.

So starting with the base rate can help. What else?

On Wharton Moneyball the hosts discussed the 2020 Olympics and noted that the United States (men especially) have underperformed compared to years past. Is it culture? Training? Commitment?

“Is the U.S. doing badly or are other countries proportionally better?” – Shane Jensen, Wharton Moneyball, August 2021

Yes, says cohost Cade Massey, “Give Shane credit for the most parsimonious explanation we suggest for most any situation, try regression to the mean on for size and see if that can explain it.”

The idea behind regression to the mean is that performance varies up and down. There are many causal explanations for why this happens, think about talking heads for sports or stocks, but sometimes the mechanism is just old fashioned randomness.

But, ugh, we do no like this. Give me a reason man. So we assign reasons, which may be more comfortable than they are accurate.

We can make easier decisions. Easy decisions are designed. What designing a decision does is it shifts the information people use. Simple starter explanations like, the base rate or mean reversion, both create a decision making structure that will often help people get pointed in the right direction first. Start in the ballpark or base rates and then move towards your unique situation. Start with mean reversion as the mechanism and the adjust for other factors.


Mean reversion, said Cliff Aeneas (Bloomberg) is basically value investing, “they’re almost synonymous.”