The Brownian motion assumption is rather strong but not required for the conclusion. Consider the stock market, which famously has heavy-tailed, bursty returns. It happens all the time for the S&P 500 to move 1% in a week, but a 10% move in a week only happens a couple of times per decade. I would guess (and we can check) that most weeks have >0.6x of the average per-week variance of the market, which causes the median weekly absolute return to be well over half of what it would be if the market were Brownian motion with the same long-term variance.
Also, Lawrence tells me that in Tetlock’s studies, superforecasters tend to make updates of 1-2% every week, which actually improves their accuracy.
The Brownian motion assumption is rather strong but not required for the conclusion. Consider the stock market, which famously has heavy-tailed, bursty returns. It happens all the time for the S&P 500 to move 1% in a week, but a 10% move in a week only happens a couple of times per decade. I would guess (and we can check) that most weeks have >0.6x of the average per-week variance of the market, which causes the median weekly absolute return to be well over half of what it would be if the market were Brownian motion with the same long-term variance.
Also, Lawrence tells me that in Tetlock’s studies, superforecasters tend to make updates of 1-2% every week, which actually improves their accuracy.