how do they determine/calculate dead peoples I.Q?

[1R0N M4N XL]

https://financesonline.com/13-most-intelligent-people-in-the-history-of-the-world/

how do they come with these IQ numbers... most of these people have been dead for more than 100 years ago before I.Q test were invented/created by some Frenchman?

[brennus dux gallorum]

By personality traits that characterize people with particular iq levels

Of course these estimates are not accurate