Does a negative cross correlation show high or low similarity?
I am programming some image processing techniques which requires comparing the similarity of two sub images. I'm using a the normalised cross correlation metric which returns a value between -1 and +1. Should I be taking the absolute value of this as my similarity measure or does negative cross correlation imply a poor similarity?
Answer
Negative and positive correlation is meaningful and all depends on your application. Let me to make it more clear. Suppose you have three datasets (e.g. A : Age, B : Hair and C : Height). Suppose that the correlation between A and C is positive (0.98), so it mean that by increasing the age, it is expected to be taller). However, we you calculate the correlation between A and B, you find that it is negative! What does it mean? It means by increasing age, you expect to have less hair! So, as you can see, positive correlation means a parallel increasing/decreasing in both dataset, while negative correlation means having two opposite trend which can be meaningful, because base on the negative correlation you can expect to have more hair when you are kid!