Recent empirical studies imply that most appraisal error is nonrandom, which suggests that strategies that advocate portfolio assembly over individual property selection may be defective.
Each step of the appraisal process involves an unknown amount of estimation error. The combination of these errors is unlikely to produce a perfect, error-free estimate of value. Thus, appraisal error is virtually unavoidable. Investors need reasonable estimates of value when buying, selling, or retaining commercial property, so an unknown amount of appraisal error adds uncertainty to the decision-making process. Despite the uncertainty, investors have learned to make allowances for appraisal error in their decision-making processes. The way in which real estate investors interpret appraisal errors has a material effect upon the decisions that they make. In particular, the predominant belief among real estate professionals is that appraisal error is random. This belief materially influences investor attitudes toward portfolio management and the valuation process itself. Lack of understanding of the relative magnitudes of random and nonrandom components of total appraisal error has consequences for optimal portfolio strategies. For example, investors who deem the bulk of total appraisal error to be random may reasonably conclude that error in estimates is beyond their control or influence. To minimize total portfolio valuation error, such investors may assemble large, diverse portfolios even though the cost of owning an array of properties of various types and in various locations is expensive. On the other hand, if the bulk of total appraisal error is nonrandom, investors would do better to pay attention to improving value estimates on each property rather than hoping that the errors in values of a large pool of properties will offset one another. In particular, investors should