In a positive correlation, as one variable increases, so does the other variable, and as the first decreases, so does the second. A negative correlation is the opposite. As one variable increases, the other variable decreases, and as the first decreases, the second increases.
The length of an iron bar increasing as the temperature increases is an example of a positive correlation. An example of a negative correlation is that the volume of gas decreases as the pressure increases. If there is no relationship between the two variables, they are said to have no correlation or zero correlation. For example, if one variable changes and the second variable stays constant, these variables are said to have no correlation.