# Correlation and Similarity

Suppose I have two vectors, both of which are probabilities of something (sum to 1). Under what circumstances will correlation (say Pearson corr.) and similarity (say cosine sim.) differ largely?

I found one question one this but it does not touch one this specific question.

• Both correlation and cosine similarity/distance are measures of similarity/distance. Is your question specifically about Pearson's correlation and cosine similarity? – Pieter Aug 5 '17 at 22:09
• @Pieter Yes, let's take Pearson corr and cosine sim. as an example. – Zhiya Aug 6 '17 at 13:01

They are in general pretty different in magnitude.

Let $v = (1/n, 1/n, 1/n, ... 1/n)$ be an n-dim probability vector.

Then $v$ has a correlation of 1 with itself, but its cosine similarity score with itself is only $1/n$.

Another example:

let $v = (.1, .9)$ and $w = (.9, .1)$. Then the correlation between v and w is -1 but the cosine similarity between them is .18.

Any probability vectors will have a non-negative cosine similarity score, but may have a negative Pearson correlation.

• Thanks! But how should I interpret when two probability vectors have high cosine similarity (e.g., >0.9) but negative Pearson correlation (around -.2)? – Zhiya Aug 14 '17 at 3:49
• Can you give me an example of two such probability vectors? Thanks – Rachel Kogan Aug 19 '17 at 19:44
• Sorry for my late reply. Here it is: u=(0.086455, 0.081696, 0.07411, 0.082895, 0.086318, 0.085153, 0.087153, 0.085869, 0.083544, 0.088836, 0.079762, 0.078209); v=(0.084606, 0.090245, 0.103638, 0.06136, 0.091619, 0.077643, 0.062884, 0.082069, 0.081827, 0.090805, 0.079354, 0.093951) – Zhiya Sep 8 '17 at 16:15
• No. The dot product of those vectors is approx .08, and the correlation is approx 90%. – Rachel Kogan Sep 23 '17 at 18:29
• I dont think so. Here are the result: scipy.stats.pearsonr(u,v) gives (-0.43695954986949204, 0.15550155101173249); from sklearn.metrics.pairwise import cosine_similarity; cosine_similarity(u,v) gives array([[ 0.98606257]]) – Zhiya Sep 25 '17 at 13:48