One of the criticisms of the CAPM is that beta is derived from volatility of prices of publicly traded companies, which differ from non-publicly companies in liquidity, marketability, capital structures and control. Other aspects such as access to credit markets, size, and management depth are generally different, too. The rate build-up method also requires an assessment of the subject company's risk, which provides valuation of itself. Where a privately held company can be shown to be sufficiently similar to a public company, the CAPM may be suitable. However, it requires the knowledge of market stock prices for calculation. For private companies that do not sell stock on the public capital markets, this information is not readily available. Therefore, calculation of beta for private firms is problematic. The build-up cost of capital model is the typical choice in such cases.