Non-correlation of investment assets may seem to be complicated, but it is very important to the long-term success of an investment portfolio. To understand non-correlation, we must first understand what we mean when assets are correlated. Investment assets are referred to as being highly correlated when they show a tendency to vary together. For example, U.S. stock classes--large, medium and small-- tend to increase and decrease in value together. For many years, large international stocks were not considered to be correlated to U.S. stocks, but they are now 92% correlated.