Hi there, I'm Dr. And I hope you've been enjoying our discussion on portfolio construction so far as we develop basic portfolio measures such as portfolio expected return and portfolio volatility. The examples we have looked at have tried to illustrate the key insight behind diversification. Even though assets may individually be risky, meaning that they may have uncertain future returns, as long as there is some lack of parallelism in their fortunes in the economy, combining them into a portfolio creates diversification and reduces risk. This is the key insight. And the degree of parallelism is what we measure using statistical measures like co-variance and correlation in our formulas. Okay so you learned that diversification is good. Now is there a limit to how much we can diversify. In this lecture you will learn key concepts such as what is systematic risk, what is idiosyncratic risk. The risk we can diversify away, the idiosyncratic risk. And the risk that we can't diversify away, what we call the systematic risk. And finally what are the sources of these types of risk. Okay, so let's first remember the effect of correlation on diversification. So think of the following example. Suppose two assets are moving exactly in tandem. Would holding them together reduce risk. In the extreme case, think of the extreme case, that they are exactly in tandem such that they are perfectly correlated. So, they are correlation coefficient is plus one, would it created diversification. No, all right. Putting them together will not reduce risk, there is no diversification benefits. Now, typically however right. The fortunes of different companies, or assets, or businesses do not always move exactly and completely in tandem, right. Which leaves us with lots of potential for being able to diversify risk by putting them together in a portfolio. And now here is the real kicker. The correlation between two assets does not even need to be negative to achieve risk reduction for diversification, right. Even if it's 0.5 there is some moderate risk reduction, right. Even when it's zero there is some risk reduction. So as long as the assets are less than perfectly correlated which is the case with a correlation coefficient of one. Adding another security or asset class will always reduce risk. Okay, so now let's move to the light board and let me illustrate this point to you graphically.