K 2 is the index of independence test, K 2 >; At 6.635, we say that these two events are related with more than 99% certainty, k 2.

K2 = n (ad-BC) 2/(a+b) (a+c) (c+d) (b+d) where n = a+b+c+d.

And @xyza_808' s answer is completely wrong!

R is the correlation coefficient and R 2 is the correlation index. The closer these two numbers are to 1(- 1), the stronger the correlation will be.