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As far as I know, correlation is used between two numerical columns. However, columns like `vehicle_1`

', `vehicle_2`

… and `cause_vehicle_1`

, all contain nominal data. Then, how is it possible to calculate the correlation of nominal data?

Also, what is a null correlation, I’m familiar with “correlation” but never heard of “null correlation”? what is it?

Update: While searching the web for “null correlation”, I encountered the “nullity correlation matrix”. Can some please explain what does it mean how to interpret it?