Before a comet or asteroid hit Jupiter last year, astronomers had calculated that an impact like that would happen on average every 350 years.
How did they arrive at that number? The lay-person might (amusingly) think that they had simply taken the only two other known impacts (Shoemaker-Levy in 1994 and a possible impact observed by Giovanni Cassini in 1640), subtracted the two, and got the value of 354 years, but not so!
According to arXiv, arriving at that number involved taking those two impacts, adding "crater counts on Jupiter's large moons and various theoretical calculations", and THEN deciding it was 350 years.
I am suspicious.