Combining securities into portfolios reduces risk. When combined with other securities,a portion of a stock's variability in return is canceled by complementary variations in the return of other securities.Some firms represented in the portfolio may experience unanticipated adverse conditions (e.g.,a wildcat strike).However,this may well be offset by the unexpected good fortune of other firms in the portfolio. Nevertheless,sine to extent stock price (and returns) tend to move in concert,not all variability can be eliminated through diversification.Even investors holding diversified portfolios are exposed to the risk inherent in the overall performance of the stock market (for instance,the stock market crash of October 1987). Thus, it is convenient to divide a security's total risk into that portion that is peculiar to a specfic firm and can be diversified away (called unsystematic risk) and that portion that is market-related and nondiversifiable (called systematic risk)