Weak or no correlation exists if there is no such apparent relationship.
Covariance
It is a measure that quantifies the strength and direction of a relationship between a pair of variables.
cov(x,y)=n1i∑n(xi−x)(yi−y)
Correlation Coefficient
The correlation coefficient, or Pearson product-moment correlation coefficient is another measure of the correlation between data. You can think of it as a standardized covariance.