This document analyzes Sharpe's ratio as a measure of investment fund performance. It discusses Sharpe's ratio in the context of portfolio theory and as an approximation of a utility index. The document proposes some modifications to Sharpe's ratio to avoid inconsistent assessments and better approximate a utility index. It establishes six postulates regarding utility theory in the presence of risk to provide a conceptual framework. The document notes two cases where Sharpe's ratio may not function properly and proposes an alternative variation to address situations where expected return is less than the risk-free rate of return. It applies the various performance measures to a sample of Spanish investment funds.