In any case, there is no single ideal roofing or flooring or insulation because every context is different. The directories mentioned above, as well as other materials databases, usually try to provide guidelines for selection, based on the design goals of the construction project. For some projects, price may not be a major concern, making environmental quality a top priority; other projects may have to be done as environmentally as possible under tight budgetary constraints. Product information is no less important to each case. Although some of the environmental product directory projects have attempted to provide overall environmental guidelines for product selection, most have focused on best case choices. That is, the directories are not concerned primarily with evaluating all building materials, but mainly listing those that can be considered the greenest. To be listed, therefore, is something of an endorsement. Because new products and are emerging all the time, new materials knowledge continues to evolve, the tendency is for the directories to raise the bar, and increase their criteria of what is green. Authors of the Green Spec directory, for example, have given notice that the next edition of the directory will purge a number of current product listingssimply because products of higher environmental quality are becoming commercially available. Here are some examples of positive criteria employed by the green products directories to select their products: products with low embodied energy and that are produced with fewer resources (e.g. recycled flyash concrete).