What is correlation? It is a statistical measure of the relationship between series of numbers -- such as financial returns. Before considering an example of the effects and benefits of diversification, we should define some terms --

• Correlation coefficient (or covariance) is a measure -- from +1 to -1 -- that measures the degree of the relationship, or correlation, between two series.
• Positive correlation means that two series tend to move in the same direction -- when one goes up, the other goes up; when one goes down, the other goes down. This means the correlation coefficient is positive. If the series is perfectly positively correlated the coefficient is +1. For example, you and your shadow are perfectly positively correlated.
• Negative correlation means that two series move in opposite directions -- when one goes up, the other goes down; and vice-versa. If two series are perfectly negatively correlated, the coefficient is -1. For example, when you clap your hands their motion is perfectly negatively correlated -- we hope.
• Uncorrelated series lack an interaction between their returns -- when one goes up, the other might go up or might go down. The correlation coefficient is zero.