Correlation is a technique which measures the strength of association between two variables. Both the variables and may be random, or it may be that one variable is independent (non-random) and the other is correlated or dependent. When the changes in one variable appear to be linked with the changes in the other variable, the two variables are said to be correlated. When the two variables are meaningfully related and both increase or both decrease simultaneously, then the correlation is termed as positive. If an increase in any one variable is associated with a decrease in the other variable, the correlation is termed as negative or inverse.
Suppose marks in mathematics are denoted by and marks in statistics are denoted by . If small values of appear with small values of and large values of come with large values of , then the correlation is said to be positive.
If stands for marks in English and stands for marks in mathematics, it is possible that small values of appear with large values of . This is a case of negative correlation.