Let's say I have two vectors of $1$ and $-1$, and I want to know how similar these two vectors are. Is the use of the cosine similarity coefficient, justifed in this case?
Solved – Cosine similarity
binary datacosine similaritysimilarities
binary datacosine similaritysimilarities
Let's say I have two vectors of $1$ and $-1$, and I want to know how similar these two vectors are. Is the use of the cosine similarity coefficient, justifed in this case?
Best Answer
Let $x, y\in\{-1,+1\}^k$. Then their cosine similarity is
$$ \cos\theta =\frac{x\cdot y}{\|x\|_2\|y\|_2}=\frac{x\cdot y}{k} $$
since
$$ \|x\|_2=\|y\|_2=\sqrt{k}. $$
And
$$ x\cdot y = \#\{i\,|\,x_i=y_i\}-\#\{i\,|\,x_i\neq y_i\}$$
simply counts the number of concordant minus the number of discordant pairs. So your cosine similarity is simply this number scaled by $k$ to $[-1,+1]$.
I'd say this kind of similarity makes perfect sense.