Why does a portfolio's risk depend more on how its stocks move together than on how volatile each one is?
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Moving northwest on the mean–standard deviation graph
Sketch the graph with across and up, mark the preferred direction, and record that picking a single stock such as Merck or GM gives no clear answer.
Portfolio return versus portfolio mean
Write as the accounting identity, take expectations to get , and record the distinction between the two lines.
Portfolio variance as an table sum
Expand into its terms , draw the table with variances on the diagonal, and state Markowitz's point about small or negative cross products.
Counting variances against covariances
Count the diagonal variances and the off-diagonal covariances, and state the conclusion about which of the two dominates portfolio risk as grows.