A stock's beta is more relevant as a measure of risk to an investor who holds only one stock than to an investor who holds a well-diversified portfolio.

Beta measures a stock's volatility relative to the overall market. For an investor who holds only one stock, the stock's beta can give an indication of how much risk is associated with that specific stock. However, for an investor who holds a well-diversified portfolio, beta becomes less relevant because the overall risk of the portfolio is already spread across multiple stocks with varying betas. In this case, the investor would be more concerned with the portfolio's overall beta or the risk of the portfolio as a whole rather than the beta of any one individual stock.