Great Filter: put a number on it or sit down

Order-of-magnitude sketch, not a paper. Stars in the galaxy ~ 10^11. Fraction with planets: ~1. Fraction in habitable-ish zone over Gyr: 10^-1 to 1. If abiogenesis is not vanishingly rare, we should expect a lot of kitchens. Intelligence plus expansion is the expensive line item. Either n_civilization is tiny, lifetime is short, or expansion is blocked. Pick a decade and a probability. 'It's complicated' is not a model.