Keeris, Willem, and Ruben Langbroek. "Determination of the Value of Property Investments by Means of the Performance Potential and Risk-Return Improvement Ratio." In 14th Annual European Real Estate Society Conference. ERES: Conference. London, UK, 2007.

Keeris, Willem, and Ruben Langbroek. "Measuring the Stability of Performance by Means of the Volatility Ratio." In 14th Annual European Real Estate Society Conference. ERES: Conference. London, UK, 2007.