the key insight in the CAPM was that risk should be measured relative to a fully diversified portfolio of risky assets such as common stocks. the simple adage "don't put all your eggs in one basket dictated that investors could minimize their risks by holding assets in fully diversified portfolios. An asset's risk was not measured as its individual risk. Instead, the asset's contribution to the risk of a fully diversified or market portfolio was what mattered. This risk, usually called systematic risk, was measured by the beta coefficient