Variance is another statistical tool which tells how much a data is diverging from the mean. It is the mean of the squared differences. Where the differences refer to the difference between the mean and the individual values. Its scale is in the range of square of the maximum value.
Standard deviation is the tendency of the data to differ from the mean. Mean, median and mod estimate the midpoint of the data standard deviation tells how much the data is spread out. Standard deviation is the square root of the variance.
Covariance measures the extent to which two variables, say x and y, move together. A positive covariance means that the variables move in tandem and a negative value indicates that the variables have an inverse relationship. While covariance can indicate the direction of relation, the correlation coefficient is a better measure of the strength of relationship.
Correlation coefficient measures the degree to which two variables move together. Its value ranges between -1 and 1. -1 indicates perfectly negative relationship, 1 shows a perfectly positive relationship and zero means there is no linear relationship between the variables. Correlation doesn’t necessarily mean causation.