Recently, as I was following the 🇺🇸 US elections 🇺🇸, I got puzzled by the fact that days after the elections happened some betting odds of Biden winning the elections were still below 90%, while FiveThirtyEight published a probability of 90% before the election actually happened, when way less information was known.

I decided to spend my Sunday of COVID lockdown illustrating some facts about forecasting, like the martingale property.