We read with interest the paper by Michiels et al on the prediction of cancer with microarrays and the commentary by Ioannidis listing the potential as well as the limitations of this approach (February 5, p 488 and 454). Cancer is a disease characterized by complex, heterogeneous mechanisms and studies to define factors that can direct new drug discovery and use should be encouraged. However, this is easier said than done. Casti teaches that a better understanding does not necessarily extrapolate to better prediction, and that useful prediction is possible without complete understanding (1). To attempt both, explanation and prediction, in a single nonmathematical construct, is a tall order (Figure 1).

A letter report issued by the Government Accountability Office with an abstract that begins "Mobility is critical to the nation's economy. Projections of future passenger and freight travel suggest that increased levels of investment may be needed to maintain the current levels of mobility provided by the nation's highway and transit systems. However, calls for greater investment in transportation come amid growing concerns about fiscal imbalances at all levels of the government. As a result, careful decisions will need to be made to ensure that transportation investments maximize the benefits of each federal dollar invested. In this report GAO identifies (1) the categories of benefits and costs that can be attributed to new highway and transit investments and the challenges in measuring them; (2) how state, local, and regional decision makers consider the benefits and costs of new highway and transit investments when comparing alternatives; (3) the extent to which investments meet their projected outcomes; and (4) options to improve the information available to decision makers. To address these objectives, we convened an expert panel, surveyed state departments of transportation and transit agencies, and conducted site visits to five metropolitan areas that had both a capacity-adding highway project and transit …