What Correlation Means
Correlation describes the statistical relationship between the price movements of two assets. It is usually expressed as a number between -1 and +1. A correlation of +1 means two assets tend to move in the same direction at the same time. A correlation of -1 means they tend to move in opposite directions. A correlation near 0 means their movements show little consistent relationship.
Correlation does not measure how much an asset moves, only how consistently its direction relates to another asset. Two assets can be highly correlated even if one is far more volatile than the other. For the size of movements, investors typically look at volatility alongside correlation.
Why It Matters
Correlation sits at the heart of diversification. Combining assets that do not move in lockstep can smooth a portfolio's overall ride, because losses in one holding may be partially offset by gains or smaller losses in another. If everything in a portfolio is highly correlated, holding many positions may create only the appearance of diversification while offering little real protection.
Correlation also shapes asset allocation and hedging decisions. Investors often mix asset classes, such as stocks and bonds, partly because their historical correlations have differed across market environments.
A Simple Example
Imagine two stocks, A and B. Over a year, whenever A rises, B usually rises too, and when A falls, B usually falls. Their correlation might be around +0.9. Now imagine stock A and a bond fund: sometimes they move together, sometimes apart, giving a correlation near +0.2. A portfolio holding A and the bond fund would likely experience smaller combined swings than a portfolio holding A and B, even if the individual assets are similar in risk.
Common Mistakes
- Assuming correlation is fixed. Correlations change over time and often rise sharply during market stress, exactly when diversification is needed most.
- Confusing correlation with causation. Two assets moving together does not mean one drives the other.
- Ignoring the time frame. Daily, monthly, and yearly correlations for the same pair of assets can look very different.
- Treating low correlation as no risk. A low-correlation asset can still lose value; it simply loses value on a different schedule.
What to Verify Before Acting
Before using correlation figures in a portfolio decision, check the data period and frequency behind the number, and whether it reflects recent market conditions. Consider how correlations behaved during past downturns, not just calm periods. Review how a new holding fits your overall asset allocation and your plan for rebalancing. Portfolio analysis features vary by platform, so compare what different providers offer using a broker comparison tool before relying on any single correlation dashboard.