Find the C-factor of a vote

R, 23 bytes

function(o,v)mean(o!=v)

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The challenge boils down to computing the proportion of values in v different from o (i.e. mean(xor(o,v))). We can therefore avoid using abs.

Jelly, 3 bytes

ạÆm

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Literally just "absolute difference to mean".

ạÆm  Main link
ạ    Absolute difference
 Æm  Arithmetic Mean

If you invert the arguments you can invert the atoms.

APL (Dyalog Unicode), 9 8 5 bytes

≠⌹⊢=⊢

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Anonymous train. Thanks to @Adám for a byte saved, and thanks to @ngn for 3 bytes!

How:

≠⌹⊢=⊢ ⍝ Anonymous Train
    ⊢ ⍝ The right argument (⍵)
  ⊢=  ⍝ Equals itself. Generates an array of 1s
≠     ⍝ XOR left (⍺) and right args; generates ⍵ or (not ⍵), depending on ⍺.
 ⌹    ⍝ Divide these matrices.